Timing Light, Food, & Exercise for Better Sleep, Energy & Mood | Dr. Samer Hattar

Andrew Huberman 0:00 (Music) Welcome to the Huberman Lab Podcast, where we discuss science and science-based tools for everyday life. I'm Andrew Huberman, and I'm a professor of neurobiology and ophthalmology at Stanford School of Medicine. Today, I have the pleasure of introducing Dr. Samer Hattar as my guest on the Huberman Lab Podcast. Dr. Hattar is the Chief of the Section on Light and Circadian Rhythms at the National Institute of Mental Health in Bethesda, Maryland. Dr. Hattar has many important discoveries to his name. He was one of a handful of groups that discovered the light-sensing neurons in the eye that set the circadian clock. This was a fundamental discovery made in the early 2000s that has led to an enormous number of additional discoveries on how light regulates our sleep, our immune system, our mood, mental health, metabolism, feeding, and many other important processes. If ever there was somebody who understands how all of these processes interact and can inform best practices for our daily behaviors, it's Dr. Hattar. During our discussion today, Dr. Hattar answers questions that are absolutely essential for us to know about our health and well-being. For instance, how to align our sleep schedule with our activity schedule, such as exercise, and how to align light, activity, and exercise with our feeding rhythms. He presents a new model of how light, activity, and feeding rhythms converge to support optimal health, and when those are not aligned correctly, how our mental and physical health can suffer. It's a discussion that is rich with scientific mechanism, made clearly of course, so everybody can understand, as well as specific protocols to deal with shifts in day length, shifts in activity, and in order to optimize sleep, metabolism, and well-being of various kinds. I learned so much from Samer, as I always do. He is an absolute wealth of knowledge on all things related to light and circadian rhythms, physiology, and neuroscience. I don't think you'll find anyone else as knowledgeable about these topics as Samer, and so I'm delighted that he joined us here on the podcast to share this information.

Andrew Huberman 2:17 Before we begin, I'd like to emphasize that this podcast is separate from my teaching and research roles at Stanford. It is however part of my desire and effort to bring zero cost to consumer information about science and science-related tools to the general public. In keeping with that theme, I'd like to thank the sponsors of today's podcast. Our first sponsor is Roka. Roka makes eyeglasses and sunglasses that are of the absolute highest quality. I've spent my career working on the science of the visual system, and I can tell you that one of the things that our visual system has to contend with is adjusting so that when we go from a very bright area to a dim or shadowed area, we can still see things clearly. Roka eyeglasses and sunglasses were designed with the science of the visual system in mind, and so they make those transitions seamless. You always see things with crystal clarity. Another great thing about these glasses is that they're very lightweight, so you don't even really remember that they're on your face, and they won't slip off if you get sweaty. The glasses were designed initially for running and for cycling and for active wear, but they work great for that, and they work great, and they also happen to look great, for work, if you go out to dinner, for social settings, so they can really be worn in essentially any circumstances. If you'd like to try Roka glasses, you can go to Roka, that's ROKA.com, and enter the code Huberman to get 20% off your first order. That's ROKA.com, enter the code Huberman at checkout. Today's podcast is also brought to us by InsideTracker. InsideTracker is a personalized nutrition platform that analyzes data from your blood and DNA to help you better understand your body and help you reach your health goals. I've long been a believer in getting blood work done, for the simple reason that many of the factors that impact our immediate and long-term health can only be detected in a quality blood test. The problem with a lot of blood tests out there, however, is you get numbers back but you don't know what to do about those numbers specifically. Roka solved that problem at a number of levels. First of all, they make getting the blood tests very easy. They'll come to your house if you like, or you can go to a local clinic. Second of all, once you get your numbers back, there's a very easy to use dashboard where you can identify obviously what the numbers are, but also the various things that you can do to bring those numbers into the ranges that you want, through either behavioral practices like exercise, through nutritional practices, or supplementation, et cetera. So they made the whole thing very easy start to finish, in a way that allows you to best direct your health goals. If you'd like to try InsideTracker, you can visit InsideTracker.com/Huberman to get 25% off any of InsideTracker's plans. Just use the code Huberman at checkout. Today's episode is also brought to us by Magic Spoon. Magic Spoon is a zero sugar, grain-free, keto-friendly cereal. I am not ketogenic, meaning I don't follow a purely ketogenic diet. I tend to fast in the early part of the day, I tend to eat kind of low carb-ish through the middle of the day, and then in the evening I eat carbohydrates. That's what works best for me, and allows me to feel alert all day long and to sleep really well at night. Magic Spoon is a terrific snack for me because it tastes terrific, it's got some sweetness, but it doesn't take me out of that state that I want to be in during the day where I'm sort of keto-ish I would say. I'm not actually in ketosis, but I'm following more or less a low carb diet during the day, which keeps me alert, so either fasting or low carb, and Magic Spoon is consistent with that, and then as I mentioned before in the evening I do eat carbohydrates. Magic Spoon has 0 grams of sugar, 13 to 14 grams of protein, and only four net grams of carbohydrates in each serving, so I think it qualifies as low carb-ish or low carb. In addition, it only has 140 calories per serving. It's also just delicious. They have flavors like cocoa, fruity, pea- peanut butter, frosted. I particularly like the frosted one because it tastes like donuts and I particularly like donuts, although I try not to eat them too often, if ever. If you want to try Magic Spoon, you can go to MagicSpoon.com/Huberman to grab a variety pack. You can use the promo code Huberman at checkout and you'll get $5 off your order. Again, that's MagicSpoon.com/Huberman and use the code Huberman to get $5

Andrew Huberman 6:15 off. And now my conversation with Dr. Samer Hattar.Sammer, thanks for sitting down with me.

Dr. Samer Hattar 6:22 My pleasure.

Andrew Huberman 6:23 Yeah. We go way back. Uh, so you are best known in scientific circles for your work on how light impacts mood, learning, feeding, hunger, sleep, and these sorts of topics. So, just to kick the ball out onto the field, so to speak, how does light impact the way we feel? So, when I get up in the morning, I have the opportunity to interact with light in certain ways or to avoid light in certain ways. I have the opportunity to interact with sunlight or with artificial light. Maybe you could just wade us into what the relationship is between light and these things like mood and hunger, et cetera.

Dr. Samer Hattar 7:04 Sure. So, I mean, you do appreciate the effect of light for vision. So, when you wake up in a beautiful area, beautiful ocean, light is essential. The sunrise, the sunset, blue sky, beautiful mountains. So, that's your conscious perception of light but light has a completely different aspect that is independent of conscious vision or image forming functions, and that's how it regulates many important functions in your body. I think the best that is well studied and well known is your circadian clock, and the word "circadian" comes from the word "circa" which is approximate and "dien" is day, so it's an approximate day. Why is it an approximate day? Because if I put you or any other human being who have a normal circadian clock in a constant conditions with no information about feeding time, about sleep time, about what time it is outside, you still have a daily rhythm, but it's not exactly 24 hours. So, it will shift out of the solar day because it's not exactly 24 hours and hence the name circadian.

Andrew Huberman 8:11 So, just to ask a quick question about that. When you say you have this t- uh, s- about 24-hour rhythm, how does that rhythm show up in the tissues of our body?

Dr. Samer Hattar 8:20 Great. So, great question. So, it shows up at every level that we know and we studied. It shows up at the level of the cell, it shows up at the level of the tissue, and it shows up at your behavior. The most obvious for you is your sleep-wake cycle. You sleep and, uh, you're awake and sleep at a 24-hour rhythms, and if you measure the sleep-wake cycle of humans who are maintained in constant conditions, you will see that the period length of the sleep rhythm on average is more than 24 hours. In humans, it's 24.2 hours. So, you'll be drifting 0.2 hours every day out of the solar day if you don't get the sunlight. So, the s- the sunlight adjusts that approximate day to an exact day, so now your behavior is adjusted to the light-dark environment or the solar day.

Andrew Huberman 9:08 Okay, so if I understand correctly, if I were to go into a cave-

Dr. Samer Hattar 9:11 Yeah.

Andrew Huberman 9:11 ... or I were to be in constant light-

Dr. Samer Hattar 9:13 Yeah.

Andrew Huberman 9:14 ... and I didn't close my eyes (laughs) -

Dr. Samer Hattar 9:15 Yeah.

Andrew Huberman 9:15 ... in constant light, that I would still sleep in one coherent bout.

Dr. Samer Hattar 9:21 Yes.

Andrew Huberman 9:21 And I would still be awake for more or less one coherent bout.

Dr. Samer Hattar 9:24 Yes.

Andrew Huberman 9:24 Maybe a nap. But the total duration of my day, so to speak, would be a little bit longer than 24 hours.

Dr. Samer Hattar 9:34 Perfect.

Andrew Huberman 9:34 But if I'm in, in a condition like most people are where the sun goes up and the sun goes down and I have some understanding of that sunrise and sunset, then-

Dr. Samer Hattar 9:43 You don't have to have the understanding. You don't have to have conscious understanding. You have the detection.

Andrew Huberman 9:48 Right.

Dr. Samer Hattar 9:48 So, circadian photoentrainment is the word we use, entraining the circadian clock to the photic environment, is completely subconscious. You're not aware of it. It's not like vision where... or image forming where you actually know what you're looking at. So, it's all hypothalamic. It's part of the brain that is not consciously driven, so you actually do not know when it happens or when it doesn't happen, and that what we'll get into when I tell you why light affects your mood and why sometimes people don't know how to deal with light to improve their mood, for example.

Andrew Huberman 10:20 Okay, so this is a subconscious vision.

Dr. Samer Hattar 10:22 Yes.

Andrew Huberman 10:22 Okay. Uh, before you tell us about how light impacts mood, I, I'm curious, what is the relevance of adjusting this clock from a little bit longer than 24 hours to 24 hours? I mean, it seems like a small difference, 24 hours and 40 minutes or 24 hours. Like, what, who, what, what's, what's the relevance? I mean, why should we care about that short difference?

Dr. Samer Hattar 10:44 So, let's do the math. If you shift out 0.2 hours a day, in five days you're shifting out one hour, so you're literally one hour off in your social behavior in five days. In 10 days, you're two hours off. And if you're a- an organism that is living in the wild shifting out of the right phase of the cycle, you could either miss food or you could become food, so it's really essential for survival. I think it's one of the strongest aspect of survival for animals to have the anticipation and the adjustment to the solar cycle.

Andrew Huberman 11:16 And for humans as well. When you say animals, I'm assuming that applies to us.

Dr. Samer Hattar 11:19 Absolutely. Yeah.

Andrew Huberman 11:21 I see. So, even though it's just a short, uh, bit longer than 24, if you... if that accumulates over days, then you could find yourself very much out of phase with the rest of your, your species essentially.

Dr. Samer Hattar 11:33 Yeah. So, let's say it's 0.2 hours, so in five days it's one hour. In 25 days, it could be five or six hours. It... you could be in New York and you're feeling as if you traveled from New York to London. So, you will be having jet lag in New York even though you didn't do a jet lag travel. So, it's very important for the adjustment. And if we have time, maybe we could talk about why this is important for seasonality, because also it allows animals to anticipate the change in season. And the more you're high in, in the north or the south, the more that this- these weather changes occur very harshly and you have to be ready for them, and that happens in, in, in us as well. <inaudible .

Andrew Huberman 12:15 All right. Well, we, we will definitely get into seasonality. Um, okay, so we've got this subconscious vision that aligns us with the turn of the earth.How does that work? Wh- what's the, what is the, the machinery that allows that to happen and w- how does that machinery work?

Dr. Samer Hattar 12:33 Yeah. So, we know, we knew that in mammals, including us, we are mammals, humans, that the eyes are required for this function. So, if humans are born without eyes or the optic nerves are damaged, humans are not able to adjust to the solar cycle. So, we know that the eyes are required. And since we thought we knew about the eyes a lot before 2000, we thought that-

Andrew Huberman 12:55 When you say before the year 2000? Mm-hmm.

Dr. Samer Hattar 12:57 Before the years 2000, yes.

Andrew Huberman 12:59 Yep.

Dr. Samer Hattar 12:59 We thought it's these photoreceptors in your retina that allow you to see. So, in, in, in the human retinas, there are two types of photoreceptors. They are called rods and cones because of their shapes. And these rods and cones simply take the photon energy, which light is made of, and they change it in a way to an electrical signal that allow us to build the image of the environment in our cortices.

Andrew Huberman 13:23 Subconsciously.

Dr. Samer Hattar 13:24 Consciously in this situation-

Andrew Huberman 13:25 I see.

Dr. Samer Hattar 13:25 ... 'cause it's vision, right? It's image forming vision. It's a, it's a visual cortex and association, associative cortices which allow you to build conscious perception of the environment. However, people have found, including me with the work of David Berson and Ignacio Provencio, that there is a subset of ganglion cells. The ganglion cells are the cells that leave the retina, their axon, leave the retina and project to the brain. So, these were thought to only relay rod and cone information from the light environment to the brain. We found that a small subset of these ganglion cells are themselves photoreceptors that were completely missed in the retina. And these are the photoreceptors that relay light environment subconsciously to the areas in the brain that have and house the circadian clock or the circadian pacemaker, which adjust all the clocks in our bodies to the central brain clock, that allows them to entrain to the 24-hour light-dark cycle.

Andrew Huberman 14:27 Incredible. So, in the year- as I recall,

Andrew Huberman 14:30 'cause I was a graduate student at the time, in the year 2000, there was this landmark discovery made by you, Iggy Provencio, David Berson, and others, that these cells exist that can communicate day and night information to the brain.

Dr. Samer Hattar 14:42 Yeah.

Andrew Huberman 14:42 And this very small subset of s- cells. Since then, I've heard, but maybe you can confirm or refute, that this system that connects the eyes to the rest of the brain is actually the most ancient form of vision, that this is probably the form of vision that some early version of human beings had before they had pattern vision, before they could see colors and shapes and motion and, and all that. And that the, the w- that the same cells that perform this role are actually similar to insect eyes. I think I heard David Berson say once that we actually have a little bit of fl- the fly eye in our eye. What's he talking about?

Dr. Samer Hattar 15:20 Yeah. So, it's, it's really interesting actually, because these same IPrgCs we discovered they contribute a little bit to image formation, and now work from Tiffany Schmidt specifically have proven that they do contribute to image forming functions. But they contribute to very, um, limited aspect of image formation. So, it fits your hypothesis that these are an ancient photoreceptors. The other thing that adds to that hypothesis is that they are expressed in cells that don't have any modification that make them look like photoreceptors. So, our- the photoreceptors that I told you about that are important for vision image formation, they have very specialized structures that allow them to back these structures with photopigments. These are the photo-detecting proteins. So, they could detect a high sensitivity of photons that pass through them. These IPrgCs don't ha- uh, these new photoreceptors don't have these specialized structures, so they just really need a lot of light. At the time, we thought they need a lot of light to be activated. So, that's why we think they are ancient and that's why I think they adjust to ancient functions that are as important as regulating your body circadian clock to the solar environment, to solar day, or to the light-dark cycle.

Andrew Huberman 16:42 So, you, you mentioned IPrgCs-

Dr. Samer Hattar 16:43 Yeah.

Andrew Huberman 16:43 ... intrinsically photosensitive. So, these are cells that connect the eye and the brain that behave like photoreceptors essentially.

Dr. Samer Hattar 16:49 Right.

Andrew Huberman 16:49 And then you mentioned, uh, melanopsin which is the actual pigment that-

Dr. Samer Hattar 16:53 Yes.

Andrew Huberman 16:53 ... converts the light into the electrical signal-

Dr. Samer Hattar 16:56 Yeah.

Andrew Huberman 16:56 ... more or less. And my understanding is that melanopsin was identified first in frog melanophores.

Dr. Samer Hattar 17:03 Yes.

Andrew Huberman 17:04 So, does that mean that we have like little pieces of frog skin in our eyes?

Dr. Samer Hattar 17:08 So, honestly, David Berson say you have a fly in your eye because it sounds better. The more accurate I think is that you have a frog skin in your eye. It's not as catchy, but really melanopsin, really the name melanopsin, is from melanocyte opsin. So, it's melanopsin because it was found in the frog melanocytes. You know the frogs can change their color depending on light? And melanopsin drives this response. So, when Ignacio Provencio first discovered these opsins in frogs, luckily he was smart enough to see if they are expressed in the frog eye. They were expressed in the frog eye, and in what appears to be retinal ganglion cells, which I told you the one that connect the eye to the brain, he had the insight to go and see if they are expressed in the monkey eye. And he found that they are also expressed in what appears to be retinal ganglion cells, and really that what opened the field wide open. Then David Berson did the seminal experiment where he went to the brain where the central oscillator, the, the oscillator that drives circadian rhythm in the brain called the suprachiasmatic nucleus that has been known for many years to receive retinal input, and he labeled the cells that project there, and then he found that even if you destroy rods and cones, you could get light responses from these cells. So, you could imagine, he, he nearly fainted when he saw that these cells can respond independent...... completely in the absence of rod and cone input.

Andrew Huberman 18:32 Yeah, I'll never forget reading those papers in 2000, 2001. I was at the meeting in DC when Iggy show- Ignacio, Iggy, we call him Iggy, showed this image of this, basically what is frog melanophores in the human eye.

Dr. Samer Hattar 18:47 Yes.

Andrew Huberman 18:47 And everyone was like, "Oh, my goodness, this is the thing."

Dr. Samer Hattar 18:49 Yeah.

Andrew Huberman 18:49 Um, and I want to get into how light actually can control circadian rhythms in a moment.

Dr. Samer Hattar 18:54 Absolutely.

Andrew Huberman 18:55 But I think, um, it's worth mentioning now that, uh, people who are pattern vision blind, so people who cannot see and no conscious vision, but have eyes, many of them still have these cells, these melanopsin intrinsically photosensitive cells, and can, uh, essentially match or entrain, as we say, onto the, uh, light-dark cycle.

Dr. Samer Hattar 19:16 In fact, they possibly have no problems in circadian photo entrainment, they'll have normal sleep-wake cycle-

Andrew Huberman 19:21 But they're totally blind.

Dr. Samer Hattar 19:22 ... but they are totally image blind. And what's really interesting is that, and this story I heard from Chuck Czeisler, so I'll give him credit, that some of these people who are image blind, usually they get dry eyes, and they give them a lot of pain. And doctors used to think, "Oh, th- since they are image blind and they're getting dry eye, why don't you just remove their eyes? They're not using them anymore." And the minute they would remove their eyes, they start having cyclical sleep problems indicating that now they are not entraining to the light-dark cycle, and are having cyclical jet lags when their clock shifts through the light-dark cycle.

Andrew Huberman 19:57 Hmm. That's really interesting, and, uh, I hear from a number of blind people, uh, you know, in my various, uh, aspects of my job, and they, a lot of them have issues with, with sleep, I think, in part, because they don't realize that they too need to see light at particular times of day or night in order to-

Dr. Samer Hattar 20:13 Absolutely.

Andrew Huberman 20:14 ... master their schedule.

Dr. Samer Hattar 20:15 Absolutely.

Andrew Huberman 20:15 Well, I think that's a perfect, um, segue for us to talk about how light and viewing light can impact our sleep-wake rhythms. And then we will move into some of the other ways in which light can impact other forms of bodily function.

Dr. Samer Hattar 20:30 Yeah, so I, I love the way you set it up because one of the m- most interesting and difficult aspect of trying to educate people about light effect on subconscious vision is that it's subconscious. So we're all aware of what we think is intensity because we see the room, but, you know, if you talk to people who know how to take photographs and stuff like that, they know that the intensities varies greatly, but our system, because we have to see the same way in very bright conditions and very dim conditions, we're not very good at estimating int- intensity consciously. So, when you try to tell people about intensity, you really struggle because they think they know intensities, but they really don't be-

Andrew Huberman 21:12 You mean light intensity?

Dr. Samer Hattar 21:13 Light intensity.

Andrew Huberman 21:14 Yeah.

Dr. Samer Hattar 21:14 So, so the, the cones themself have an incredible ability to adapt to different light conditions, so you can see at all different conditions, otherwise it'd be a disaster, you know? If you don't change the, the setting on your camera and you go from inside the room to the outside, it becomes completely white, you don't see anything. So if your cones don't adapt to the environment, then you're not going to be able to see in this room and in, on the beach, right? But the problem is your, your ipRGCs, the cells that we talked about, they measure intensity pretty well. They really know what intensity is. They have a very good linear measurement of intensity. They don't adapt as well, they don't adapt actually that much, to be honest. So that tells you that subconsciously the system is used to measuring light intensity in a natural environment, because when you are now in a natural environment, you don't have, you know, industrialized lighting, then you, you know, your system is functioning very well. But now, when we change these environments, we could really mess up ourselves, so you have to teach people how to understand intensity, and that's something that you have to explain to people, and I think I love to do it myself, I do it in what is called the lowest amount of light required to allow you to see comfortably, so you have to do this as an, a fun experiment.

Andrew Huberman 22:36 Okay, so explain to me how this goes, er, and maybe we could break it up in- the day into, um, three or four parts. So let's say-

Dr. Samer Hattar 22:43 Right.

Andrew Huberman 22:44 ... uh, assuming that most people wake up in the morning-

Dr. Samer Hattar 22:46 Mm-hmm.

Andrew Huberman 22:46 ... as opposed to night shift workers, et cetera, we could talk about later.

Dr. Samer Hattar 22:49 Right.

Andrew Huberman 22:49 But you wake up in the morning, so let's divide the day into quarters. What is, what is the proper way to interact with light in the first part of the day?

Dr. Samer Hattar 22:59 So I honestly think the easiest thing is waking up. Get as much light as you can.

Andrew Huberman 23:05 Into your eyes?

Dr. Samer Hattar 23:06 Yeah, it's really nice. Your system is primed. If you're entrained, it's primed to get light. The sun should be out. Most animals in the wild, they actually seem to track the, the sun. The sun has a huge influence on life on Earth. It's actually, life on Earth is because of sun. So that's easy. In the morning when you wake up, you need light. Just get the light.

Andrew Huberman 23:26 Okay. So what, what is the behavioral practice that, that you recommend? Does it... let's say somebody is in a condition where there's a lot of cloud cover-

Dr. Samer Hattar 23:33 Yeah.

Andrew Huberman 23:33 ... is it important to get outside?

Dr. Samer Hattar 23:35 So I have to tell you, the cloudiest day is going to be much more brighter than your room. You could ask any photographer. A cloudy day, unless it's really dark, dark clouds, usually cloudy days have much more bright outside than inside the room, even when you have good lighting inside the room. So I think in the outside is usually, even when it's cloudy, you're going to get enough intensity to help you adjust your cycle to the day-night cycle.

Andrew Huberman 24:01 So how, how long do you- a j- these are general rules of thumb.

Dr. Samer Hattar 24:05 Absolutely.

Andrew Huberman 24:05 But how long do you recommend people go outside?

Dr. Samer Hattar 24:07 So if you do it daily, you possibly need very f- if, if you do it daily, because, remember, this thing is going to happen on a daily matter, so I would say 15 minutes.

Andrew Huberman 24:14 So it, right, so the clock is tracking it on a regular basis?

Dr. Samer Hattar 24:16 Absolutely. It's, it's photon counting, it's tracking, I would say 15 minutes. If you don't do it daily, you may want to increase it, and we'll talk about when you travel what you could do, but, but yeah, 15 minutes should be fine. You do it more, it doesn't hurt.

Andrew Huberman 24:29 And through a window, I was... My understanding is that through a window, it dramatically decreases the amount of light energy coming in.

Dr. Samer Hattar 24:35 It, it depends of how, you know, how thick the windows are and how dark they are. So, it's... But it's also nice to go outside and to, to feel the season.

Andrew Huberman 24:44 Sunglasses off.

Dr. Samer Hattar 24:46 I don't use sunglasses. I-

Andrew Huberman 24:47 Yeah, but you have the, your Jordanian form-

Dr. Samer Hattar 24:50 Yeah.

Andrew Huberman 24:50 ... of pigment, you know?

Dr. Samer Hattar 24:51 Yeah.

Andrew Huberman 24:51 So-

Dr. Samer Hattar 24:52 That's right.

Andrew Huberman 24:52 Yeah. Whereas my li- eyes are very sensitive, right?

Dr. Samer Hattar 24:54 Yeah. No, so I-

Andrew Huberman 24:55 So-

Dr. Samer Hattar 24:55 But I personally, you know, if I'm in the shade or if it's not incredibly bright, I try to, especially in the morning, but I'm a- also an early person, so we have to differentiate between early and-

Andrew Huberman 25:05 What time do you wake up?

Dr. Samer Hattar 25:06 I wake up at 4:30 in the morning.

Andrew Huberman 25:07 But the sun isn't out yet.

Dr. Samer Hattar 25:08 It's not out yet, so it's-

Andrew Huberman 25:10 So, what do you do? You turn on artificial lights?

Dr. Samer Hattar 25:12 I usually don't turn on artificial light because I know the sun is going to come up eventually, but that's why I don't like the change in the timing that they do. Do you-

Andrew Huberman 25:21 Wait, but what do you do between 4:30 AM and 7:00 AM?

Dr. Samer Hattar 25:23 I mean, I look at my computer-

Andrew Huberman 25:25 Oh, I see.

Dr. Samer Hattar 25:26 ... and my phone.

Andrew Huberman 25:26 Okay.

Dr. Samer Hattar 25:26 So, possibly I get enough light.

Andrew Huberman 25:28 Okay.

Dr. Samer Hattar 25:28 But, but in reality, I mean, as long as you let your body get the morning sunlight, which I think is really, to me, and I... There's no evidence, but to me, this is... If you look at all animals, plants, this morning sunlight seems to be very important. And I, I, you know, we don't have experiments to show it, but I have a gut feeling that it has a huge impact on humans.

Andrew Huberman 25:51 Well, Jamie Zeitzer's lab at the Stanford S- Sleep Lab has shown that these early morning light flashes-

Dr. Samer Hattar 25:56 Mm-hmm.

Andrew Huberman 25:57 ... can adjust the total amount-

Dr. Samer Hattar 25:59 Ab- absolutely.

Andrew Huberman 25:59 ... of sleep that one will get, makes it easier to get into sleep.

Dr. Samer Hattar 26:02 Absolutely, absolutely.

Andrew Huberman 26:02 Okay, so-

Dr. Samer Hattar 26:03 And, and, and, uh, Ken Wright also did this beautiful camping experiments that showed if you-

Andrew Huberman 26:09 Right, maybe you should, could describe those 'cause those are-

Dr. Samer Hattar 26:11 Oh, yeah.

Andrew Huberman 26:11 ... beautiful experiments.

Dr. Samer Hattar 26:11 They are beautiful experiments. He sh- he, he took these, you know, college students that had a late onset of sleep and late waking time, and then he said, "Let's go camping and just don't use any artificial light, and you could go to sleep as, as, as late or as early as you want and wake up as late, as early." And he found a huge shift in their sleep pattern just by exposing them to the light-dark cycle. I mean, so it has-

Andrew Huberman 26:36 And it lasted?

Dr. Samer Hattar 26:37 And it lasted.

Andrew Huberman 26:38 Even after they came back, right?

Dr. Samer Hattar 26:39 Exactly.

Andrew Huberman 26:39 I think it was two days of camping reset the circadian clock.

Dr. Samer Hattar 26:41 Seven days, but it, it lasted, yeah, it's pretty amazing. Yeah, it's really incredible.

Andrew Huberman 26:46 Okay, so get bright light of some sort early in the day, ideally sunlight, even on a cloudy day, it's going to be-

Dr. Samer Hattar 26:54 Absolutely.

Andrew Huberman 26:54 ... brighter than indoor light.

Dr. Samer Hattar 26:55 Yeah.

Andrew Huberman 26:55 Okay.

Dr. Samer Hattar 26:55 So that's easy.

Andrew Huberman 26:56 Okay, so then-

Dr. Samer Hattar 26:57 And, and the other thing that I would like to mention to people. If it, if you think it's very dim outside, let's say it's very cloudy, stay longer. So remember, intensity is only one component. Duration is also important because remember that the circadian system is not like the image system. In the image system, you have to change every second because you're looking at different objects, you have to change your perception. But for the circadian system, it's trying to figure out, "Where am I in the day-night cycle?" So the more you give them the information, the better you are. So if it's very bright, you don't need a lot because it's clearly going to make you fire like crazy. But if it's not bright, stay longer, stay for one hour, you know, have your coffee outside or something like that. It's just going to help.

Andrew Huberman 27:41 I think you said something extremely important, which is that this circadian system is trying to figure out when you are in time-

Dr. Samer Hattar 27:48 Exactly.

Andrew Huberman 27:48 ... not where you are in space.

Dr. Samer Hattar 27:50 Sorry, I said where you are in time.

Andrew Huberman 27:51 Yeah.

Dr. Samer Hattar 27:51 I meant when you are.

Andrew Huberman 27:52 Oh, no, no, I wasn't correcting you. I just meant that it... I think fundamentally that's the, the, the incredible thing about this system-

Dr. Samer Hattar 27:59 Absolutely.

Andrew Huberman 27:59 ... that you have this clock, this 24-hour clock in your brain, but it, it needs to be synchronized to the outside. So, could we go a little deeper into this circadian setting behavior and come up with some general rules of thumb? So let's say it's a very bright day-

Dr. Samer Hattar 28:15 Mm-hmm.

Andrew Huberman 28:15 ... extremely bright.

Dr. Samer Hattar 28:16 Mm-hmm.

Andrew Huberman 28:17 You know, no clouds, the sun's out. You said 10 minutes, 15 minutes.

Dr. Samer Hattar 28:22 And I'll tell you, if you're sensitive, don't... You don't even have to go in the sun. You could be in the shade. There's going to be so many photons out there in the shade; it's going to be perfect. You don't even have to see the sun. You don't have to have the sun. You know, it's great for vitamin D. That's a different story. You could do this for your skin and, and, you know, protect your skin. That's not my area of expertise, but for, for that effect on the circadian system, as long as you're outside in the shade and it's sunny day, 10 to 15 minutes should be ample amount.

Andrew Huberman 28:53 Okay, and then let's, let's say it's kind of overcast.

Dr. Samer Hattar 28:57 Mm-hmm.

Andrew Huberman 28:58 Um, you know, it's not particularly bright or there's, you know, solid cloud cover, but you know, obviously the, the sun is out, but, uh, you're... It's not as bright. How long do you think it would take to set the clock?

Dr. Samer Hattar 29:07 10 To 15 should be sufficient. Stay for half an hour, stay for 45 minutes. If it's very dark around, yeah, stay for longer.

Andrew Huberman 29:14 Okay, and if, um, for some reason one finds themself very far north and it's very, very dense, uh, cloud cover, how long and at what point should somebody consider using an artificial light source to mimic the sunlight?

Dr. Samer Hattar 29:30 Yeah. Honestly, this is where we don't have a lot of information still because this is where we're going to discuss this maybe in more detail that if you put humans in artificial conditions, the circadian system is very sensitive to light. But in reality, in the real environment, light also is affecting other aspects that are independent of the setting of the circadian pacemaker.

Andrew Huberman 29:55 Okay.

Dr. Samer Hattar 29:55 And these which we call the direct effect of light on mood, for example. So that is very hard to figure out what intensity you need to use, and we haven't done enough experiments because the system has been discovered just recently.

Andrew Huberman 30:07 Mm-hmm.

Dr. Samer Hattar 30:08 But I would say if you use bright light in the morning, and I, I mean, it's hard for me to give numbers, it can get complicated.

Andrew Huberman 30:16 Mm-hmm.

Dr. Samer Hattar 30:16 Um, but yeah, I mean, if you're... Honestly,

Dr. Samer Hattar 30:20 if you're that far north and you're in the winter and you want to get... Make sure you don't... Use these light boxes. I, I would suggest that personally, but that's it.

Andrew Huberman 30:28 I use, um, it's actually not designed for circadian setting, but I have a, a 930 lux light pad-

Dr. Samer Hattar 30:35 There you go.

Andrew Huberman 30:35 ... that I bought and I bought it, um, they're very affordable compared to the, uh, dawn simulating lights-

Dr. Samer Hattar 30:40 Yeah, you don't need-

Andrew Huberman 30:40 ... which are, which are-... quite expensive, frankly.

Dr. Samer Hattar 30:43 Yeah.

Andrew Huberman 30:43 And I, I put it there and so I u- just basically when I wake up in the morning, I use that until the sun comes out. And then I make sure once the sun is out, I go outside. But I keep that thing on all day and I don't know if that's good or bad. Is it good or bad?

Dr. Samer Hattar 30:56 I, honestly, I don't think being exposed to bright light in the day is going to ever be bad, because really if you're outside in the day, un- unless, you know, the worst that's going to happen is the temperature is very high, your body's going to say, "Don't dehydrate and go to sleep." So you, you could tell actually sometimes when it's very hot the more you get exposed to bright light, the sleepier you will feel in the afternoon which is counterintuitive. Um, so you-

Andrew Huberman 31:20 Mm-hmm. And that's to protect us you think against dehydration?

Dr. Samer Hattar 31:23 I think if you think about the human evolution from near the equator in the m- uh, between noon and a certain time in the afternoon, it would have been very hard for you to maintain physiological homeostatic function being active at this very high temperature time. So, I think napping was a way, that's why I think it has a major function, which is still... Napping was a way to somehow take you away from that dangerous zone and maybe that's why people in the north they say in the winter, "We can't wake up in the morning," 'cause they don't have this long light so they sleep it more at night. But in the summer they say, "We feel like we can't go to sleep. We have to put all these dark curtains..." So I think, you know, venturing that up that much north, up north has been... Came up with problem because evolution was used to a certain light environment that was completely changed with human. With other animals, I think that lived there longer, um, they have come up with very interesting adaptation of actually measuring even very small changes in the light in- the light intensities that still occur, so... And even if you're near the- the- the poles, even though it's always light, but there is a change in the light intensity across the day-night cycle. So, your system-

Andrew Huberman 32:38 Mm-hmm.

Dr. Samer Hattar 32:38 ... if it's linear, and remember I told you that ipRGCs are incredibly linear, can still measure, "Oh, this is lower light than higher light," if- if- if the organism has the ability to do that.

Andrew Huberman 32:49 I see. You know, it's interesting. I've- I've spent so much time, uh, learning from you, uh, fortunately about these cells and yet I never really appreciated until now how on the one hand they are tracking the amount of light to understand when we are in time relative to the 24-hour cycle.

Dr. Samer Hattar 33:07 Mm-hmm.

Andrew Huberman 33:07 But also that you keep mentioning this- this linear measurement of intensity, that they really are trying to figure out when we are in time by measuring the intensity of light. And of course, the sun is the most intense source of light-

Dr. Samer Hattar 33:19 Exactly.

Andrew Huberman 33:19 ... available to us. So okay, so I think we've, um, have nailed down that first part of the day.

Dr. Samer Hattar 33:25 Exactly.

Andrew Huberman 33:26 Basically it's get 10 to 30 minutes depending on how bright it is.

Dr. Samer Hattar 33:29 Right.

Andrew Huberman 33:29 And try and do that as r- as often as possible to give the system a regular s-

Dr. Samer Hattar 33:32 Daily is the best. This system is really about... And- and you'll see that even for the effect on depression, it's about multiple days. It's just... So you don't have to worry if you missed it one day, you know, stay longer if you want. But if you're in a hurry and you want to do other stuff,

Dr. Samer Hattar 33:48 that's a great recommendation.

Andrew Huberman 33:49 Mm-hmm. So you might want to compensate with some extra time if you miss a day or two?

Dr. Samer Hattar 33:53 Absolutely.

Andrew Huberman 33:53 And this is why I- I've heard you say before, it's entirely possible to get severely jet lagged without traveling.

Dr. Samer Hattar 33:59 Absolutely.

Andrew Huberman 34:00 Simply by staying in, being on your phone too much-

Dr. Samer Hattar 34:03 Absolutely.

Andrew Huberman 34:03 ... not getting the sunlight.

Dr. Samer Hattar 34:04 And you saw this during the pandemic.

Andrew Huberman 34:06 Mm-hmm.

Dr. Samer Hattar 34:06 A lot of people mentioned that their sleep-wake cycle suffered a lot. Uh, because if you're not going out and if you're staying at home and you don't have big windows and you're waking late, waking up late, and then you're using very bright light 'til late at night, your body's going to shift. And now your day is going to start instead of like really when the sun comes up, let's say, at 6:00 in the morning, it's going to... So your day is going to start at 11:00 in the morning. That's what your body's going to think is the beginning of the day. So, then you're not going to be able to sleep at 10:00 at night because now that's really for- for your body is completely different timing. And you could see this happen during the pandemic at a very high scale. People got delayed in their sleep-wake cycle a lot.

Andrew Huberman 34:52 And, um, there is this idea of chronotypes that we all each intrinsically have a- a best rhythm of either being a morning person, uh, you-

Dr. Samer Hattar 35:01 Absolutely.

Andrew Huberman 35:01 ... called yourself an early person, or a night owl, or more of a kind of standard, you know, to bed around 10:30, up around 7:00 type, um, thing. And the f- and I think there are now good data, correct me if I'm wrong, from the National Institutes of Mental Health and elsewhere showing that the more we deviate from that intrinsic rhythm, the more mental health issues and physical health issues start to crop up.

Dr. Samer Hattar 35:23 Oh, so there is great data on this and there is couple of things that complicate this. The first is the people who usually are late, they tell you that the society doesn't accommodate them.

Andrew Huberman 35:33 What, l- by late, um, what- what do you mean? People that wake up late-

Dr. Samer Hattar 35:35 That means-

Andrew Huberman 35:35 ... and go to sleep late?

Dr. Samer Hattar 35:36 Go to sleep late and wake up late. They u- they have an overwhelmingly higher level of depression and failures.

Andrew Huberman 35:43 Mm-hmm.

Dr. Samer Hattar 35:43 I mean, clearly, I mean, the reason that people say, "Sleep early, wake up earlier," better because human notice that people who wake up, go to sleep early and wake up early they do better in life. They notice that.

Andrew Huberman 35:56 They just perform better.

Dr. Samer Hattar 35:57 They perform... But the question is, is- is that intrinsic to the system or is that society? Because society start things usually early or late. That's a hard question to answer.

Andrew Huberman 36:05 We discriminate against late risers.

Dr. Samer Hattar 36:07 We, in a way we discriminate, right? But the other explanation is Ken Wright's experiment. The- these late riser, if they were truly chronotypically late, why would they shift so easily when you put them in the li-... If you were really chronotypically late and there is a phase relation between the light-dark environment and your circadian clock, then doing this camping experiment should not have caused much changes because it's not that, you know, you- you... Light is going to affect you in a certain way, it's that this is the relationship that your body decided that, "I'm a late sleeper, late waking."So, I, um, honestly, I'm still un- un- unable to figure out how much of this late waking up is controlled by the light environment and how much is intrinsic. I'm sure there are differences, but are they as big as we see in the environment? Because you have people that go off to sleep at 7:00 PM and wake up at 1:00 AM. These are clearly advanced phase-

Andrew Huberman 37:06 So, people that go to sleep at 7:00 PM and wake up at 1:00 AM.

Dr. Samer Hattar 37:09 Yeah.

Andrew Huberman 37:09 And feel good doing that.

Dr. Samer Hattar 37:11 I'm not so sure they feel good, but lot of the time you talk to people they say they are high achievers, but they suffer because, you know, they, they wake, they go to 7:00 PM, wake up, advanced phase sleep syndrome they call it, they, they call it a syndrome, but, you know, but then you have people who would not be able to sleep till 5:00 AM and, and not be able to wake up till 3:00 PM, right? And I'm not so sure that the circadian system is that variable in the human population. I mean, clearly there are maybe some genetic factors that make a small percentage of, like, everything with a bell shape. But I think most of the time, the light environment may play a role, and once, as you, as we've talked about, this is a long-term effect of light. Once you get into a rhythm, and I, I don't mean it as a pun, in reality once you get into a rhythm, it's hard to break out of that rhythm because if you start sleeping late and waking up late, you're not getting the morning sunlight.

Andrew Huberman 38:06 Right.

Dr. Samer Hattar 38:07 And so, you, you're just gonna be late. And if you're like me waking up early, you're getting the morning sunlight, you're getting what Zeisler said, uh, um, I said his last name wrong. Um, the one in Stanford who did the, the-

Andrew Huberman 38:21 Oh, uh, Jamie Zeitzer.

Dr. Samer Hattar 38:22 ... the Zeisler.

Andrew Huberman 38:22 Yeah.

Dr. Samer Hattar 38:22 Yeah.

Andrew Huberman 38:23 He actually worked for...

Dr. Samer Hattar 38:25 Zeisler.

Andrew Huberman 38:26 Zeisler. So, Zeisler and Czeisler. Yeah, there are a lot of Zs, Zs, and Ahs in those, in those, in their names.

Dr. Samer Hattar 38:30 Yeah.

Andrew Huberman 38:30 Both phenomenal scientists.

Dr. Samer Hattar 38:31 Yeah.

Andrew Huberman 38:32 What, what, what

Andrew Huberman 38:33 it seems to me is the case is that the only way to really know if you're meant to be an early bird, as they call it, an early person or a late person, or somewhere in between, is to get morning sunlight and figure out whether or not that makes you feel better.

Dr. Samer Hattar 38:48 And to understand, to be educated about how to measure intensity, how to measure... I put it between quotation 'cause you either get a measuring device, but you cannot depend on eye- h- on your eye to measure intensity.

Andrew Huberman 39:03 Okay. So, how do we do that? 'Cause you're, you keep coming back to this, so I, that tells me that's important.

Dr. Samer Hattar 39:08 It's very important.

Andrew Huberman 39:08 So obviously, so there are apps, like free apps, um, like, uh, Light Meter where you can walk around and hold the button down and see how many lux, you know, are in the environment.

Dr. Samer Hattar 39:17 These are complicated because you have to point them to specific regions.

Andrew Huberman 39:21 Right. So, so how, how do people start to develop an intuitive sense of the measurement of intensity?

Dr. Samer Hattar 39:27 Yeah. I think... At one point I posted on Instagram how I keep my nighttime at home, and I found out that my night vision is very strong. So, I found out that I, especially in the winter, I only need candlelight, so I literally use these tea lights and I put like 15 or 20 of them.

Andrew Huberman 39:45 How romantic.

Dr. Samer Hattar 39:46 And it's so nice. I could see, it clearly doesn't affect my circadian system.

Andrew Huberman 39:51 You and your cats.

Dr. Samer Hattar 39:52 And, and my wife.

Andrew Huberman 39:53 And your wife, of course.

Dr. Samer Hattar 39:53 It's just great. It's just great, right? But I don't expect people to have the same night vision as me. So, the simp- I mean, I tell people, "Do the experiment." So, if you put three or four lights in your room, switch two, sit for 15 minutes. You have-

Andrew Huberman 40:09 Switch two off.

Dr. Samer Hattar 40:10 ... switch two off, let's say you're using five, and see after 15 minutes you will not recognize you switched these two off. My gut feeling is that most people would need at least 10 times less light than they use at night to see. The problem people use it, because most of the time they didn't see the morning sunlight, they are actually hungry for light without their knowledge. So, they come switch all these lights on but at the wrong time because they woke up late.

Andrew Huberman 40:36 Okay. Now, now I understand. So, so this morning light viewing goes way beyond-

Dr. Samer Hattar 40:41 Absolutely.

Andrew Huberman 40:41 ... setting your clock.

Dr. Samer Hattar 40:43 Absolutely.

Andrew Huberman 40:43 It's also a way to determine how m- how little light you need later in the day.

Dr. Samer Hattar 40:47 Exactly.

Andrew Huberman 40:48 And we, and we're going to talk about this in a moment, but how little light you get later in the day is a very strong determinant of things like when you will wake up, whether or not you wake up feeling refreshed, et cetera. Let's, let's-

Dr. Samer Hattar 40:59 And that's why-

Andrew Huberman 41:00 Yeah.

Dr. Samer Hattar 41:00 ... I'm going to break it on your show, Andrew, that I'm going to tell you I think there is something else that people need to think about which is the tripartite model. That this model incorporate three components which we talk about in details, that allows us humans and all animals to incorporate the circadian clock and its relation to light, the homeostatic drive, and the direct effect of the environment which includes stress, light, all kind of stuff. They have to be incorporated together. If you think... That's what I think right now. If you think of one alone, you will always miss something.

Andrew Huberman 41:37 Okay.

Dr. Samer Hattar 41:38 When you think of them as a whole, things really become clear. It's actually quite amazing.

Andrew Huberman 41:43 Okay. Well, we will definitely want to hear about your tripartite theory and, and go into detail about these homeostatic mechanisms. I want to make sure that for people who are, um, thinking now, I'm sure, about light and how it impacts them. So, the morning light viewing behavior I like to think we've tacked down clearly.

Dr. Samer Hattar 42:01 Absolutely.

Andrew Huberman 42:01 And thank you for that because there's so much information out there and, um, I've tried to relay that information. Of course, you're my primary source for all things circadian, as well as Jamie and others, of course, um, Matt Walker, but, um, I think you've made that very, very clear.

Andrew Huberman 42:16 Now, let's say I've gotten my morning sunlight, okay, right, or my bright artificial light, and throughout the day you said to get a lot of light. So, I'm working at my desk, maybe I'll, I'll go out during the day a few times but I'm working at my computer, I'm doing things. Is there anything about light viewing in the middle of the day that people should keep in mind? Or can they just sort of freestyle it depending on what they're doing? Most people are not, uh, you know, in a dark room-

Dr. Samer Hattar 42:41 Yeah.

Andrew Huberman 42:41 ... um, throughout the day.

Dr. Samer Hattar 42:42 So, my gut feeling, if you got your morning sunlight...... you walk from your car slowly, or you walk to work, you didn't wear sunglasses when the lights were still dim in the morning, that you could s- freestyle it. That even if you don't get a lot of light, there is a way to just, you know, in the day, you don't have to just worry about getting a lot of bright light. But f- personally, I like to do that, so I go out at lunch and, and have my lunch outside as well. This reminds the body that here it is even brighter now. But the evidence is that you could literally help your circadian clock by giving lights at dawn and dusk. But again, if you think of the tripartite model, this may be important for the c- circadian clock, but is it important for your mood? So, that's where I think you need... Or the homeostatic drive.

Andrew Huberman 43:31 Mm-hmm.

Dr. Samer Hattar 43:31 So, that's where you need to think about it. So, for the clock, for entraining your clock, you literally can entrain it only by the dawn sunlight. You actually don't need dawn and dusk.

Andrew Huberman 43:41 Okay, but-

Dr. Samer Hattar 43:42 People even forget that.

Andrew Huberman 43:42 Yeah. A- and I appreciate that you're distinguishing between circadian effects and other effects of light.

Dr. Samer Hattar 43:47 Yes.

Andrew Huberman 43:47 You're being very precise, which-

Dr. Samer Hattar 43:48 Yeah.

Andrew Huberman 43:49 ... um, is, is appreciated. Until we hear about this tripartite model, which we will cover, for, for the sake of the discussion, um, let's treat the light-viewing behavior as what are the benefits or drawbacks of viewing light for all biological purposes, not just circadian setting.

Dr. Samer Hattar 44:07 Mm-hmm.

Andrew Huberman 44:07 So, in the morning, it's clearly going to set the clock, and then during the day, if I understand correctly, the idea is to get as much bright light as you can because you're feeding, it sounds like, a sort of, um, light hunger.

Dr. Samer Hattar 44:19 Exactly.

Andrew Huberman 44:20 I see.

Dr. Samer Hattar 44:21 I love this way to put it. I think there is a weird light hunger. Considering that we're not photosynthetic organisms, there is a weird light hungers in animals that they need to measure, they need, they, they need measure, and I think that relates to the season because the whole reproduction cycle of animals is going to depend on the availability of re- food in the environment, and if you don't know when the season is going to happen, they don't have calendars, it's going to be very hard to survive. So, I think that's why we have this light hunger. That's a major hypothesis. It's not been tested.

Andrew Huberman 44:54 Interesting. So,

Andrew Huberman 44:55 then afternoon and evening start to approach. So, I've had this weird experience, maybe you can, um, psychologically or-

Dr. Samer Hattar 45:03 Mm-hmm.

Andrew Huberman 45:03 ... um, biologically diagnose me now, Samer. So, where if I go into a movie in the afternoon, like a matinee-

Dr. Samer Hattar 45:08 Uh-huh.

Andrew Huberman 45:09 ... and I come out and it's dark, I notice a significant drop in my mood and my ability to go to sleep. Whereas if I get some view of the light in the evening, it doesn't have to be the sunset, although sunsets are nice, but I get some light pulse in the afternoon, that I have no trouble whatsoever falling asleep.

Dr. Samer Hattar 45:29 And this happens in a daily... On a, on a single time to, to watch?

Andrew Huberman 45:32 Uh, more or less. Um-

Dr. Samer Hattar 45:34 Wow, that's interesting.

Andrew Huberman 45:35 And then you mentioned the camping experiment-

Dr. Samer Hattar 45:37 Yeah.

Andrew Huberman 45:37 ... where when they went camping, they're seeing the sunrise and the sunset.

Dr. Samer Hattar 45:39 Mm-hmm. Mm-hmm.

Andrew Huberman 45:41 So, what should people do in the a- afternoon/evening time in terms of their light-viewing behavior?

Dr. Samer Hattar 45:48 I mean, the best thing to do is to let the natural light creep in, into darkness, right? That would be the best. But clearly, that would be inefficient. You, you want to go home, you want to read, you want to talk to your kids, you want to talk to your family, so I think, you know, it's nice to extend the day. I don't think that's wrong, if you somehow can block that light from affecting your circadian clock. So-

Andrew Huberman 46:12 So, should people u- use blue blockers in the evening?

Dr. Samer Hattar 46:15 I personally do not like any blockers that take a single wavelength of light. Because again, if you think of a holistic, holistic approach, yes, the blue blocker is going to prevent you from affecting your circadian clock very much, but then your vision is going to be distorted because we always see in full spectrum. The sun has this beautiful spectrum, right? And then when you start seeing without the blue, s- things look yellow and it can get really weird, right? I mean, so I, I personally, I've tried the blue blocker and I couldn't even wear them. I, I thought they were just really horrendous to be honest.

Andrew Huberman 46:53 Well, uh, along the lines of blue blockers, I think a lot of people mistakenly wear them all day long.

Dr. Samer Hattar 46:58 Oh my God, that would be very bad. (laughs)

Andrew Huberman 46:59 A lot of people do that.

Dr. Samer Hattar 47:00 That would be very bad.

Andrew Huberman 47:01 A lot of people do that. They think that the blue light is bad. I think that the-

Dr. Samer Hattar 47:04 No, no.

Andrew Huberman 47:05 ... the concept of blue light being bad, um, led to the, a lot of product development.

Dr. Samer Hattar 47:10 So-

Andrew Huberman 47:10 And a lot of people are just assuming that viewing blue light is what was giving them headaches, when in fact it might have just been looking at screens-

Dr. Samer Hattar 47:16 Right.

Andrew Huberman 47:17 ... at, at, at close distance all day.

Dr. Samer Hattar 47:18 So, here is the problem, right? I mean, the blue light got the bad reputation because people who gave a pure blue light showed that it caused hugely retinal damage. But again, if you're using blue light in its pure form, it has a lot of energy because it's shorter wavelength, but we're talking about full spectrum light. There are ways now where you could change the spectrum of the light and keep it white between day and night and change the content of the color without you noticing, so you don't even have to affect your vision.

Andrew Huberman 47:49 So, h- how would you go about doing that?

Dr. Samer Hattar 47:51 So, you just lower the level of the blue light. You don't have to eliminate it.

Andrew Huberman 47:55 So, just dim the lights?

Dr. Samer Hattar 47:56 Dim the blue, but keep... Then increase the yellow, but keep all the colors in a certain white. So, you know, you could have different warmness of white, and people know how to do this. Physicists know how to do this. People who work with light know how to do this.

Andrew Huberman 48:11 Well, maybe somebody in the wellness/... Uh, I, I don't like the word, but biohacking or, um, optical community will do this. I think it's, uh, really important. I, I see so many people-

Dr. Samer Hattar 48:21 They have meetings now.

Andrew Huberman 48:21 ... wearing blue blockers.

Dr. Samer Hattar 48:22 I don't know why, why they love-

Andrew Huberman 48:24 Well, I think they're just uninformed. I think, frankly-

Dr. Samer Hattar 48:26 Yeah. And, and to be honest, it's easy, right? It's easier to explain to somebody if ipRCGs respond mostly to blue, remove blue, you'll be fine.

Andrew Huberman 48:33 Right.

Dr. Samer Hattar 48:34 Right?

Andrew Huberman 48:34 Right.

Dr. Samer Hattar 48:34 But that's not as simple as that because they also receive rod and cone input. So, you want to actually... And, you know, we could go into details that's boring for your listeners, but it also affect the adaptation properties of the whole retina. So, you don't want to do something so drastic that you take just one color of the spectrum. It just seems very counterintuitive to me.... to be honest.

Andrew Huberman 48:55 Mm-hmm. You've told me

Andrew Huberman 48:57 before as well that just because these intrinsically photosensitive circadian setting ganglion cells respond best to blue light, if the light is bright enough, because they also get input from other components of the eye, it doesn't matter if you block the blues.

Dr. Samer Hattar 49:14 Yeah. I-

Andrew Huberman 49:15 If you're looking at bright light at night, you're going to disrupt your circadian cycle.

Dr. Samer Hattar 49:17 Absolutely. And, and that's why I didn't want to go into the boring details. But themselves, the photoreceptors have a wide range of responsiveness. So, they are more sensitive to blue light, but that doesn't mean they don't respond to green light or to shorter than blue light. They respond to very, very wide spectrum with different sensitivities. So, unless you understand the system, just removing 480, I don't think is going to do anything.

Andrew Huberman 49:41 480 nanometers.

Dr. Samer Hattar 49:42 Yeah.

Andrew Huberman 49:43 Yeah. So, uh, your home is a cave at night basically-

Dr. Samer Hattar 49:48 It's a nice cave.

Andrew Huberman 49:48 ... with, with some, with some... It's a nice cave-

Dr. Samer Hattar 49:50 (laughs) Yeah.

Andrew Huberman 49:50 ... with candles.

Dr. Samer Hattar 49:51 That's right.

Andrew Huberman 49:52 Right? And you and your, your, I don't know-

Dr. Samer Hattar 49:53 And I watch TV and dim it sometimes.

Andrew Huberman 49:54 ... and your cats and your lovely wife-

Dr. Samer Hattar 49:56 Yeah.

Andrew Huberman 49:56 ... who I know who's also a phenomenal scientist in her own right.

Dr. Samer Hattar 49:59 Thank you. Yeah, she is.

Andrew Huberman 49:59 Um, she is. And, uh, but you do keep your home quite dim to dark at night.

Dr. Samer Hattar 50:05 Yeah. In fact, I did go to, to meetings with, with some of my friends who work on this, and they really struggled with me. They said, "We could have broken our legs living in the same light environment that you do." So, I am an extreme, but I measured it for myself and I asked Raziye, my wife, if she's okay with it. She also liked the dimness. Both of us can see well in, in dim conditions and, and that helps us a lot. Um, but I think you have to measure it for yourself. You really have to do... It's a very simple experiment. Just try to dim the light as much as you can. I, I call it the minimum amount of light you require to see comfortably.

Andrew Huberman 50:44 And that's how you want your environment ideally-

Dr. Samer Hattar 50:46 At night.

Andrew Huberman 50:47 ... at night.

Dr. Samer Hattar 50:47 This is what I think is the game changer. If you reach to a level where it's just barely, you're literally on the cusp of seeing uncomfortably versus seeing very comfortably, you are going to be very much better than... I don't like to make it completely dark. I think complete darkness induce anxiety in humans, to be honest, so I don't like complete darkness. Um, in fact I don't-

Andrew Huberman 51:12 Kids don't like complete darkness. They like a nightlight.

Dr. Samer Hattar 51:14 Even animal, even nocturnal animals don't like complete darkness. I mean, we have studies in animals that are nocturnal that if you put them in complete darkness for several weeks, they, they qu- they have severe anxiety and depression-like effect. So, keep the light dim. Um, you know, use red light that is very dim if you want to keep the room for sleeping. R- red light that is very dim has very small effect on, on, on circadian clock, and below 10 lux of red light literally doesn't affect sleep at all. So, there are ways to do it, it's just we need to educate the public, and I feel like you literally need a whole, uh, s- lecture to just explain it to the people how to deal with light because it's not as simple as people think.

Andrew Huberman 52:01 Well, that's what we're doing here.

Dr. Samer Hattar 52:02 Yeah.

Andrew Huberman 52:02 We're stepping through it piece by piece, and, um, and the reason we're doing that is because it's not as simple as saying, "Just block blue light or get a lot of light during the day and minimal at night."

Dr. Samer Hattar 52:10 I mean, just, just to put it in perspective, to tell it, we only have three different cones in our retina that respond to three different colors. We call them red cones for simplicity, green cones, and blue cones. Yet we have only three of these, but we could see massive palette of colors. So, that tells you something. If the system was just simply about a single color and it's, it's just removing 480 or just blue is sufficient, then we should only see in red, yellow, and blue. We shouldn't see all these different hues of color. But because the system is not that, we see all these different colors. And that's why it's important to remind people that the white light is made of many different colors. It's actually like the rainbow, that's why you see the rainbow. It's made of many colors. White light is never truly white. It's made of lot of different colors.

Andrew Huberman 53:03 It's like the Pink Floyd album cover-

Dr. Samer Hattar 53:05 Exactly. Exactly.

Andrew Huberman 53:06 ... with pink light coming through the prism.

Dr. Samer Hattar 53:06 Exactly.

Andrew Huberman 53:08 Um, so dim at night, maybe dim red light ideally or candlelight. Find that minimum required light level. What, what do you-

Dr. Samer Hattar 53:17 Test it. Just make sure when you lower the light, sit for at least 10 to 15 minutes. Let your system adapt. Because if you had a bright light and you switch it off, surely you're going to suffer because your system didn't adapt yet. It was used to very bright light. So, you want to engage your rods which take a long time to dark adapt. So, that's why I tell you just wait a little bit. Don't just switch it off, say, "I don't see." Put it on. Put it off, sit down, wait for 10 minutes, ideally 15 minutes, and then see how you see. And then, once you do that, you will notice that actually, "Ah, I could see quite well even with much less light."

Andrew Huberman 53:57 What do you

Andrew Huberman 53:58 do regarding screens?

Dr. Samer Hattar 54:00 Yeah. That's, that's the hardest thing again. I mean, there are beautiful programs that change the whole intensity and color of the screen. These could help. Dim your screen at night to the lowest part. I mean, yes, you won't see it when you wake up in the morning, but then you can increase the intensity. So, try to decrease. I mean, just what we were talking about. Think of light intensity, duration, color, and time of day. You really have to keep these four things together, right?

Andrew Huberman 54:29 We've roomed together at a couple meetings from time to time. No longer, because one of us, not to be named, has a severe snoring issue-

Dr. Samer Hattar 54:36 (laughs)

Andrew Huberman 54:36 ... that made the other one pseudo-homicidal. Um, you can guess who that was. (laughs) Uh, but-... I've seen you check your phone after dark-

Dr. Samer Hattar 54:48 Mm-hmm.

Andrew Huberman 54:48 ... once or twice. Um, and you did it by sort of pointing your phone away from you, right?

Dr. Samer Hattar 54:54 Exactly. Yeah, absolutely. And I-

Andrew Huberman 54:55 And, and actually, I'm sort of half joking, but I... and you dim it quite a bit.

Dr. Samer Hattar 54:59 Yeah, I dim it.

Andrew Huberman 54:59 I'm sort of half joking, but it actually makes sense that-

Dr. Samer Hattar 55:02 No.

Andrew Huberman 55:02 ... you know, if you shine a flashlight in your eye, it's much brighter than if you shine a flashlight on the ground.

Dr. Samer Hattar 55:05 Light only go in direct line, so if you just look on the side, most of the light is going to go this way, and you're only seeing this way.

Andrew Huberman 55:11 So, you... Okay, so, and as silly as that might seem to people listening, I mean, what it means is that getting bright light in your eyes at night is something that you really want to avoid, and, but there is the reality that-

Dr. Samer Hattar 55:22 And, and even when I check sometimes if, if I, you know, if I have something and... I check it so fast and switch it off so fast, so I'm also aware of the duration.

Andrew Huberman 55:31 Not my messages. (laughs)

Dr. Samer Hattar 55:31 (laughs) I'm, I'm, I'm also aware on the duration, right?

Andrew Huberman 55:34 Okay.

Dr. Samer Hattar 55:34 So, duration, intensity, color, and time of day. Ideally, I should not check iPhones and iPads. I don't use iPad at night because it's hard to lower it enough 'cause it's, uh, huge. But even my iPhone, I try not to use it at night. And like, once it becomes 8:30 or 9:00, I don't look at it at all. I mean-

Andrew Huberman 55:53 Unless it's World Cup or Euro Cup, in which case-

Dr. Samer Hattar 55:56 Yeah, that-

Andrew Huberman 55:56 ... Samer's on 24 hours everybody.

Dr. Samer Hattar 55:57 Right. That's only every four years. (laughs)

Andrew Huberman 56:00 (laughs) He's a big soccer fan. All right, this has been

Andrew Huberman 56:03 incredibly, um, no pun, illuminating. Um, let's talk about the relationship between light and some of these other n- non-circadian or pseudo-circadian effects, and, and we will, um, try and link those. But you had a, what I consider, absolutely landmark, beautiful paper published in Nature a few years ago, showing that if you disrupt the exposure to light, or the timing of the exposure to light, that there are dramatic effects on the stress system and on the learning and memory system. Maybe we could talk about each of those separately or together. Wh- what, what are the effects on stress and the effects on learning when light-viewing behavior and sleep-wake cycles are disrupted?

Dr. Samer Hattar 56:53 Yeah. So, just to remind you, you know that, but to remind your, uh, listeners that I was trained as a circadian biologist, so I really was indoctrinated into thinking that light has to affect the clock, which then caused all these different effects. So, that's, that's what I believed. That's my dogma. That's what would have made me really happy. And then Tara Legates and Kara Altemus joined the lab and said... and we started discussing a lot of data, and we said, "What if there is a direct effect of light that we're missing independent of the circadian clock?" So, this is not an easy question to ask, to answer, because as we've been talking all along, light affects the circadian clock, so how could you give light at different times of the day and not mess up the circadian clock? Luckily, we came up with such a way, and that's why it was important to do these experiment the way we did them, and we proved that this light-dark cycle does not disrupt the clock, there is still a circadian rhythm, and does not cause sleep, does not cause sleep deprivation. And yet, surprisingly, if you give light at the wrong time of the day, even without disrupting the circadian clock or without causing sleep deprivation, as you mentioned, you get huge mood changes in, in the organisms and you get learning deficit. So, this really... And at the time, people have really hit us hard. I mean, it, it was really hard to publish this work and, and you could-

Andrew Huberman 58:22 Well-

Dr. Samer Hattar 58:22 Yeah.

Andrew Huberman 58:22 Well, it came out in Nature, so I, I-

Dr. Samer Hattar 58:24 Yeah.

Andrew Huberman 58:24 In the end, you prevailed. Um, but I want to make sure that I understand. So, you're, you're saying that yes, there are effects of light on the circadian rhythm.

Dr. Samer Hattar 58:33 Absolutely.

Andrew Huberman 58:33 Sleep, meaning sleep and wake- wakefulness-

Dr. Samer Hattar 58:35 Yes.

Andrew Huberman 58:36 ... and their timing. However, there are direct effects of light on mood that can be dissociated from the effects on-

Dr. Samer Hattar 58:42 Absolutely.

Andrew Huberman 58:43 ... on sleep and waking. So, if I interpret that correctly, that could mean that when we view light and how much light could make us feel happier or less happy or even depressed, stressed, learning, et cetera.

Dr. Samer Hattar 58:59 Bingo. Independent-

Andrew Huberman 59:00 Even if we're sleeping and waking up at the appropriate times.

Dr. Samer Hattar 59:03 Bingo. I mean, eventually, because we're talking about the whole system, eventually when you start having the other problems, you also develop sleep problems. But you're absolutely right. And in fact, now, research from Diego Fernandez in the lab re- have found that now we know that they actually require different brain regions. So, we don't only have a theory. We don't only have a light environment that show they can be dissociated. We know that they use completely different brain regions. So, the SCN that I told you about earlier, the place where the central pacemaker is, the one that receives direct input from the retina through the ipRCGs to adjust your circadian clock, is not the area that receives the light input for mood regulation. It's a completely different brain region.

Andrew Huberman 59:49 What's the brain region called?

Dr. Samer Hattar 59:50 So, the brain region, we called it the perihabenular nucleus. Um, I'm not so sure how good or bad the name, but doesn't matter. It's the PHB. And what's really amazing, this region also receives direct input from the ipRCGs, but projects to areas in the brain that are known to regulate mood, including the ventral medial prefrontal c- prefrontal cortex, which has been studied for many years to be impacted in a human depression. So, just by, by this amazing serendipity to find that a region that is so deep in the advanced brain, like the me- the prefrontal cortex is your executive brain, one of the most elaborated in humans, to see that they receive inputs from this ancient photoreceptor was stunning to us, and told us how much we didn't understand the importance of light on human behavior.

Andrew Huberman 1:00:43 So, how does that finding inform-... daily protocols, for you or for other people? I realize you can't leap to-

Dr. Samer Hattar 1:00:51 Absolutely.

Andrew Huberman 1:00:51 ... always from one paper to daily protocols.

Dr. Samer Hattar 1:00:53 Yes.

Andrew Huberman 1:00:53 But if light indeed does control prefrontal cortex, executive function, learning, stress, uh, and mood, w- a- and let's say I'm waking up each morning-

Dr. Samer Hattar 1:01:04 That's why we came-

Andrew Huberman 1:01:04 ... and I'm sleeping-

Dr. Samer Hattar 1:01:05 That's why-

Andrew Huberman 1:01:05 ... what should I do differently?

Dr. Samer Hattar 1:01:06 That's why we came up with the tripartite model, because, yes, we could think about just adjusting the clock with lights in and being dark throughout the day, but that may not be important for your whole-

Andrew Huberman 1:01:16 Right.

Dr. Samer Hattar 1:01:16 ... physiological function. So, now if we include these other effects of light, that's why I prefer to still get a lot of light in the day. I don't want to be in very dim light condition throughout the day.

Andrew Huberman 1:01:27 I see.

Dr. Samer Hattar 1:01:28 So, even though it doesn't affect your clock, as you beautifully said, Andrew, it may affect your mood and learning and memory. It may affect your alertness level, which is going to allow you to learn better. It may affect your homeostatic drive. Maybe your homeostatic factor will go higher, so you could sleep earlier. So, it's important to think of light as stimulating all these brain regions, which means it's producing more activity, which in reality this is how people think of the homeostatic drive, that the more active you are, the more the homeostatic drive is built up, the better you sleep. So, that's why we came up with the tripartite model, because as a circadian biologist, I only thought of light through the circadian clock affecting behavior. As a sleep biologist, they only thought of the homeostatic drive affecting sleep, affecting behavior. And for people who study light for vision and other func-, they thought only of the environmental input, but now if you put them all together you get with this tripartite model where it's really mind-boggling and it makes so much sense. The organism doesn't want to depend on a single component, but if you could incorporate these three together, you could have a beautiful system that is well-adapted. So, let me tell you the sleep-wake cycle, right? So, we know there is a homeostatic drive to affect sleep. You've had beautiful talks about that.

Andrew Huberman 1:02:45 Which is basically the longer-

Dr. Samer Hattar 1:02:46 The more awake-

Andrew Huberman 1:02:47 ... you're awake-

Dr. Samer Hattar 1:02:47 ... the more awake-

Andrew Huberman 1:02:47 ... the more you want to be asleep. Yeah.

Dr. Samer Hattar 1:02:48 So, that's your homeostatic drive. We've talked about the circadian influence of sleep and the fact that light-dark cycle affect the circadian system, which eventually affects sleep, so these two components are well-understood. Now, the third factor is your direct light or environmental input. How much stress, how much light you get from there also can highly impact sleep. So, even if you have a good circadian and homeostatic drive, if you're getting w- light at the wrong time of the day, or if you're being stressed and thinking at the, then your sleep is gonna suffer. So, you have to think of the three together to have a beautiful sleep-wake cycle.

Andrew Huberman 1:03:25 Yes.

Dr. Samer Hattar 1:03:25 And that's why we came up with the tripartite model. The same thing happens with feeding. I could beautifully put it to people. Your hunger, your energy level is measured by the arcuate nucleus. Your daily intake of food is again dependent on the SCN and light-dark input. We found that if food is not available, there is yet a third input that is not dependent on the SCN, not dependent on the arcuate, depending on a completely brain, different brain regions. So, the animal can actually start looking or the human can start looking for food when it's scarce, even at time when they are not supposed to be active. So, that's how the organism think. They have to evaluate multiple inputs for them to decide what is the best physiological outcome at that moment, at that season.

Andrew Huberman 1:04:11 I see. Um, so I want to get into arcuate and feeding, but just to keep, uh, make sure we can, you know, keep our hands around this, uh, tripartite model. So, if I understand correctly, we've got the circadian influence, then you've also got the drive to sleep.

Dr. Samer Hattar 1:04:25 Right.

Andrew Huberman 1:04:25 Actually, one of the ways, um, that I think that can be best understood is if somebody ever pulls an all-nighter, they get tired around 11:00 or 12:00 or so, and then very tired around 3:00, 4:00 AM, but then even if you stay up, sometime right around 7:00 or 8:00 AM, your normal wake-up time, you start to feel alert again.

Dr. Samer Hattar 1:04:44 Exactly.

Andrew Huberman 1:04:44 And that's because the sleep drive is extremely strong, but there's a circadian rhythm that drives wakefulness in the morning.

Dr. Samer Hattar 1:04:50 Exactly.

Andrew Huberman 1:04:51 Okay, so those two are the components.

Dr. Samer Hattar 1:04:52 Right.

Andrew Huberman 1:04:53 Before we get into the feeding component, I want to talk about these direct effects of light on mood.

Dr. Samer Hattar 1:04:57 Mm-hmm.

Andrew Huberman 1:04:58 Okay? Diego Fernandez's data.

Dr. Samer Hattar 1:05:00 Mm-hmm.

Andrew Huberman 1:05:01 And this perihabenular thing.

Dr. Samer Hattar 1:05:03 Sure.

Andrew Huberman 1:05:03 So, let's just

Andrew Huberman 1:05:05 for the moment set aside the tri part of the tripartite model and just focus on, what are the direct effects of light on mood? And the way that I interpret what you've said so far is that the protocol that emerges from this, if one is trying to optimize their mood, is, yes, see light, view light I should say, early in the day in order to set your circadian clock. Maybe also in the evening as well. And, of course, avoid light at night. Uh, get it as dim as possible. However, you said it's also a good idea to get as much bright light during the day as you safely can in order to improve your mood independently of regulating your sleep-wake cycle.

Dr. Samer Hattar 1:05:43 And that's a hypothesis. Here's the problem where it's not going to be as satisfying as the circadian, is that, as you know, this brain region has been discovered very recently.

Andrew Huberman 1:05:52 Habenula.

Dr. Samer Hattar 1:05:52 The perihabenular region.

Andrew Huberman 1:05:53 But we've known about it a long time, but nobody knew what it did.

Dr. Samer Hattar 1:05:56 So, we knew about the habenula, but that's why the name is confusing. It's actually not the habenula itself. It's the perihabenula.

Andrew Huberman 1:06:02 Oh, near the habenula.

Dr. Samer Hattar 1:06:03 It's near the habenula.

Andrew Huberman 1:06:04 Why didn't you just call it the Samer Hattar nucleus?

Dr. Samer Hattar 1:06:06 I should have. I don't know-

Andrew Huberman 1:06:07 (laughs)

Dr. Samer Hattar 1:06:07 ... why (laughs) I didn't done that.

Andrew Huberman 1:06:08 Maybe 'cause if you do that, it's not okay. Okay, so for here ever after, the perihabenular nucleus, we should probably call it the Hattar-

Dr. Samer Hattar 1:06:16 (laughs)

Andrew Huberman 1:06:16 ... Hattar-Burs-

Dr. Samer Hattar 1:06:16 The Hattar-Fernandez. How about-

Andrew Huberman 1:06:17 How about Hattar-Fernandez-Bursar nucleus?

Dr. Samer Hattar 1:06:19 Yeah, that should-

Andrew Huberman 1:06:19 Okay, this is, like, nerdy, uh, science attribution-

Dr. Samer Hattar 1:06:21 Yeah.

Andrew Huberman 1:06:21 ... stuff, but I'm just gonna call it the Hattar nucleus. Um, Wikipedia, line it up.

Dr. Samer Hattar 1:06:26 (laughs)

Andrew Huberman 1:06:26 Um, okay, so this structure is taking light, and independent of sleep-... rhythms and circadian rhythms. It's driving changes in mood.

Dr. Samer Hattar 1:06:34 It's something.

Andrew Huberman 1:06:34 How does it do that? Is this through the dopamine system, the serotonin system?

Dr. Samer Hattar 1:06:37 We really, we still recently, we haven't identified this region very well. We don't know what light does to it. We don't know how it interacts. So, this is an area that is ripe for discoveries, and we're working on this right now. But that's why I said it's not satisfying. This is like the function of sleep. Why do we sleep? We know sleep is very important to us, but we still don't have a satisfying function of why do we sleep, right?

Andrew Huberman 1:07:00 I see. But the why questions-

Dr. Samer Hattar 1:07:02 We have hypotheses.

Andrew Huberman 1:07:02 ... the why questions, I think it's our, our good friend and colleague at University of Washington, Russ Van Gelder, who always says, "When somebody asks why, the, the best answer is just to say I wasn't consulted at the design phase."

Dr. Samer Hattar 1:07:12 Yeah, exactly.

Andrew Huberman 1:07:13 Right? None of us really know why.

Dr. Samer Hattar 1:07:15 No, but the point is, maybe I shouldn't have said why. What is the function of sleep?

Dr. Samer Hattar 1:07:19 It's still very hard to know. Why would or, uh, what is the reason organisms have to go offline for so long? You know, people assume it's for repair, assume it's for learning and memory, assume all kind of stuff, but there is really no clear function for sleeping. There is no clear function for sleeping. The, I mean, if you talk to people, there are hypotheses.

Andrew Huberman 1:07:42 I mean, all we know is that if you don't sleep-

Dr. Samer Hattar 1:07:44 Exactly.

Andrew Huberman 1:07:44 ... or your sleep is very fractured, you get messed up.

Dr. Samer Hattar 1:07:46 And you could die even, right? I mean, it's, it's really bad if you don't sleep. But we don't know what is the function, what is this, what is that sleep have done to organisms that couldn't have done with rest? What if you just could rest without sleeping, just sit down and rest?

Andrew Huberman 1:08:04 Well, my, my lab is trying to figure out whether or not these non-sleep deep rest protocols can compensate for s- for sleep, and-

Dr. Samer Hattar 1:08:10 So, that would be interesting.

Andrew Huberman 1:08:10 ... I mean, obviously, sleep is better, but many people are not going, getting the sleep that they need. But-

Dr. Samer Hattar 1:08:14 Right.

Andrew Huberman 1:08:15 ... but, okay. So,

Andrew Huberman 1:08:17 a- and if people are sensing that Samer and I are about to start talking over each other and arguing, that's always the goal when we talk, right?

Dr. Samer Hattar 1:08:23 That's right.

Andrew Huberman 1:08:23 Uh, unlike other scientists I interact with, when Samer and I get together, it's considered a successful conversation if we get into a big fight-

Dr. Samer Hattar 1:08:30 That's right. (laughs)

Andrew Huberman 1:08:30 ... and then go for a big meal where I pick the restaurant.

Dr. Samer Hattar 1:08:33 (laughs)

Andrew Huberman 1:08:33 Okay. So, let's talk about food and eating and appetite. You had yet another of, yes, I, I greatly admire your, your success in th- in this way, yet another incredible discovery showing that there are directs of, direct, excuse me, effects of light on appetite and feeding behavior. Can, maybe you could just summarize those results and, uh, for people.

Dr. Samer Hattar 1:08:58 And honestly, that paper is the one that allowed us to come with the tripartite model because we were thinking completely wrong about it. We wanted... This experiment, it, it'll be fun for your audience to hear why we started this experiment. Remember that when we discovered the ipRGCs, we figured if they are the only re- relay to entrain the circadian clock, then you could kill them and have an animal opposite to the one that we spoke, or human opposite to the one that we spoke about earlier, where instead of having no pattern vision and have circadian photoentrainment, we could produce an animal that have pattern vision, but no circadian photoentrainment. And-

Andrew Huberman 1:09:36 So, circadian blind.

Dr. Samer Hattar 1:09:37 Circadian blind, but pattern sighted, and we succeeded in that. The problem when you have these animals, which I've told you many times already, is that they don't adjust to the day-night cycle. So, doing experiments on them become very complicated.

Andrew Huberman 1:09:53 How, what is their behavior like if you don't have these cells? Are they awake and then asleep? Awake and then asleep?

Dr. Samer Hattar 1:09:56 They, they just drift, like the humans we've talked about. They-

Andrew Huberman 1:09:59 They think they're in Las Vegas with no clocks or watch, right?

Dr. Samer Hattar 1:10:02 They, they drift. Exactly. They do that.

Andrew Huberman 1:10:03 They stay up later every night, and they go to s-

Dr. Samer Hattar 1:10:04 They come either, depend their clock. If it's, their clock is shorter, they come in earlier. If their clock is longer, they come in later.

Andrew Huberman 1:10:10 Yeah, so they're really messed up.

Dr. Samer Hattar 1:10:11 They, they really don't adjust to the... If they were in the wild, they'll be eliminated in a second, right?

Andrew Huberman 1:10:16 (snaps fingers)

Dr. Samer Hattar 1:10:16 There is no way they'll survive. So, me and Diego started talking and we're like, "What if we use non-light entraining agent?" And what is the strongest non-light entraining agent? Food. So, we thought that the light defective animals will have more sensitivity to food entrainment because, as you know more than me, this is an area that you've worked really well on. For vision, if your image blind, your hearing and, and somatosensory get improved, right? The, the lack of vision improves your hearing and sensation. But we found actually that if you don't have the lighter system, actually you're feeding abil- the food ability to entrain the animal goes completely to the ground, completely opposite to what we predicted.

Andrew Huberman 1:11:02 So, light viewing and feeding behavior are interacting in ways that support one another.

Dr. Samer Hattar 1:11:08 And that's why we came with the tripartite model. We figured it's different than sensation of the environment. When you sense with vision, vision and hearing interact, but your vision is a real full modality. You want to see. That's what vision want to do. You want to hear. That's what hearing want to do. You want to sense. That's what sensing want to do. But for the circadian system, light, food, all these entraining agent, they somehow have to interact to keep a coherent system. You don't just assume if you remove light, this one is going to be stronger. No, they need to know each others. The light informs when the animal's going to eat.

Andrew Huberman 1:11:46 Well, what I like about this so much is that, you know, in the other, in the world outside of science in which I, I don't really exist in, but that I, I see a lot of this kind of wellness, you know, stuff with this, all this mind body integration stuff. Uh, it's interesting because people view the body more as a system, right? A system of organs that interact, as opposed to the way that standard science and medical profession is like, you work on the liver or your ear, nose and throat, or heart and lung, or brain or-

Dr. Samer Hattar 1:12:18 But that's a great way of thinking though.

Andrew Huberman 1:12:19 ... uh, you know, you know. But the biology is integrated.

Dr. Samer Hattar 1:12:22 Yeah.

Andrew Huberman 1:12:22 I mean, and so, for somebody who's interested in affecting their eating behavior, something that you are familiar with and that we will talk more about your experiences of in a moment-... how should they use light in order to adjust their eating behavior?

Dr. Samer Hattar 1:12:40 Right. So now that I've told you about all this interaction between the different inputs to the circadian clock, just you think about it as an engineer, what would be the best thing? The best thing is to know when your food times happen in the day, when should you get light, and where is your circad- when is your circadian clock in your system, right? So, if you eat at very specific times of the day, that's another signal that is telling your body, your clock, you're in a certain time of the day. So, if you're having lunch at the correct time every day and you're getting bright light, now you have two systems that are informing your clock. Your clock is going to be better.

Andrew Huberman 1:13:19 So, regular meal times?

Dr. Samer Hattar 1:13:21 Regular meal times that fit your circadian clock. So, and in fact, if you do that wh- when I started doing this and it helped me lose weight, is that I'm exposing myself to the right amount of light-dark cycle, I'm eating at regular time. It is amazing. You will be not hungry, let's say, let's say you eat at noon. You will not feel any hunger at 11:45, and then all of sudden the hunger jumps in. This is clearly not an energy issue-

Andrew Huberman 1:13:47 Right.

Dr. Samer Hattar 1:13:47 ... because it could not be that drastic.

Andrew Huberman 1:13:49 Right. No, the desire to eat is mainly driven by these, uh, these cues, these hormone cues that are very exquisitely timed to-

Dr. Samer Hattar 1:13:59 Exactly.

Andrew Huberman 1:13:59 ... sleep-wake cycle but also to light.

Dr. Samer Hattar 1:14:02 Exactly. And, you know, in the wild you could imagine why energy level through the arcuate is v- arcuate nucleus is-

Andrew Huberman 1:14:08 You should explain to people what the arcuate is 'cause I don't think we've done that adequately. The arcuate nucleus is an area of the hypothalamus that drives hunger and feeding behavior, and what we're talking about is the fact that it's taking cues from your viewing of light, believe it or not, is impacting your level of hunger. And, uh, this is a, a non-trivial way in which your timing of hunger and amount of hunger is regulated by when and how much light you view. So, let me ask you a couple practical-

Dr. Samer Hattar 1:14:38 But can I just... This is really, before you ask me-

Andrew Huberman 1:14:41 Sure.

Dr. Samer Hattar 1:14:41 ... sorry, Andrew, we said we were going to fight. But to me, is, is the, the interesting thing to think about it, in the wild when you didn't have the availability of food that we have, the arcuate plays a huge important role because if you're not, if you weren't successful in getting food, then the arcuate's going to tell you, "Look, you have to take risk and go get food because your energy level is very low." And that's great. There's tons of great research about that. But I think what's missing is the fact in humans we're not getting to a situation, most of us, we're not getting to a situation where we have low energy levels. Most of the time actually we eat not because we want to, because we are, really have low energy, but because we want to eat. So, I think that's why I feel that the timing is very important for us because we always have enough energy level for us to eat.

Andrew Huberman 1:15:31 Mm-hmm. Well, I mean, I enjoy eating so much that I'll eat just for the sensation of chewing.

Dr. Samer Hattar 1:15:35 True. True.

Andrew Huberman 1:15:35 I mean, I enjoy the taste too. (laughs)

Dr. Samer Hattar 1:15:37 True. Absolutely.

Andrew Huberman 1:15:38 And I enjoy the social aspects-

Dr. Samer Hattar 1:15:39 Absolutely.

Andrew Huberman 1:15:39 ... when those are a part of it. But I, I literally enjoy the physical act-

Dr. Samer Hattar 1:15:43 Absolutely.

Andrew Huberman 1:15:44 ... of chewing.

Dr. Samer Hattar 1:15:44 Absolutely. Uh-huh.

Andrew Huberman 1:15:46 Which explains a lot. Um, okay. So, how regular are you, or do you recommend people be about meal times? Because what I'm hearing is that light viewing behavior is pretty straightforward. Get a lot of light in the morning and throughout the day, minimize it in the evening and at night, generally speaking, for sake of mood and circadian rhythm. But for sake of regulating timing and quality, I sh- I should also say, of food intake, because people clearly make better choices about food intake when they are anticipating a meal and they aren't constantly hungry. And so the ability to regulate hunger for particular phases of the circadian cycle is, is quite valuable for all people, not just people trying to lose weight, but all people. Are we talking about down to the minute? Like, if I-

Dr. Samer Hattar 1:16:34 Absolutely.

Andrew Huberman 1:16:35 All right. Plus or... So, 12 noon is my normal lunch, let's say. Uh, plus or minus?

Dr. Samer Hattar 1:16:41 Half an hour.

Andrew Huberman 1:16:42 Okay.

Dr. Samer Hattar 1:16:42 Yeah.

Andrew Huberman 1:16:42 So, eat around, between 11:30 and 12:30?

Dr. Samer Hattar 1:16:45 If, if that's the time. And it depends. If you also do multiple meals, remember three meals, that's decision that somebody came up with. I don't know why.

Andrew Huberman 1:16:54 Nowadays people are, fewer people are doing that, I think.

Dr. Samer Hattar 1:16:56 Yeah.

Andrew Huberman 1:16:56 Gi- given our friend Satchin Panda's work.

Dr. Samer Hattar 1:16:59 Right. I mean, so you could have two meals, you could have very multiple s- meals that are distributed across your active time. I, I agree with Satchin Panda's work that try to avoid eating when your system is supposed to be relaxing, when you're supposed to be at non-active time. So, you know, limit your eating to the active time of your, of your cycle, and that seemed to be... And Joe Takashi is doing some beautiful