Essentials: Timing Light for Better Sleep, Energy & Mood | Dr. Samer Hattar
Date: 2025-08-21 | Duration: 00:30:45
Transcript
0:00 Welcome to Huberman Lab Essentials, where we revisit past episodes for the most potent and actionable science-based tools for mental health, physical health, and performance. I’m Andrew Huberman, and I’m a professor of neurobiology and of opthalmology at Stanford School of Medicine. Today, I have the pleasure of introducing Dr. Samur Hatar as my guest on the Huberman Lab podcast. And now, my conversation with Dr. Samur Hatar. Samur, thanks for sitting down with me.
0:30 >> My pleasure. >> 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 maybe you could just weigh us into what the relationship is between light and these things like mood and hunger etc. >> 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,
1:00 the sunset. So that’s your conscious perception of light. But light has a completely different aspect that is independent of conscious vision and that’s how it regulates many important functions in your body. I think the best that is well studied and wellknown is your circadian clock. And the word circadian comes from the word circa which is approximate and dn is day. So it’s an approximate day. Why is it an approximate day? Because if I put you or
1:30 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 sir. >> How does that rhythm show up in the tissues of our body? >> It shows up at every level that we know we studied. It shows up at the level of the cell. It shows up at the level of
2:00 the tissue and it shows up at your behavior. The most obvious for you is your sleepwake cycle. You sleep and you’re awake and sleep at the 24-hour rhythms. The period length of the sleep rhythm on average is 24.2 hours. So you’ll be drifting 02 hours every day out of the solar day if you don’t get the sunlight. So the the sunlight adjusts that approximate day to an exact day. So now your behavior is adjusted to the light dark environment or the solar
2:30 day. 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. What’s the relevance? I mean why should we care about that short difference? >> So let’s do the math. If you shift out 2 hours a day in 5 days you’re shifting out 1 hour. So you’re literally 1 hour off in your social behavior in 5 days. In 10 days you’re 2 hours off. And if
3:00 you’re 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. >> What is the the machinery that allows that to happen and how does that machinery work? Yeah. So we know we knew that in mammals including us we are
3:30 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 in the human retinas. There are two types of photo receptors. 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
4:00 of the environment in our cortices. However, people have found including me with the work of David Buren and Ignashi Provenio that there is a subset of ganglen cells. The ganglen 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 ganglen cells are themselves photo receptors
4:30 that were completely missed in the retina. And these are the photo receptors that relay light environment subconsciously to the areas in the brain that have and house the circadium clock or the circadium pacemaker which adjusts all the clocks in our bodies to the central brain clock that allows them to entrain to the 24-hour light dark cycle. >> So these are cells that connect the eye to the brain that behave like photo receptors essentially. I think um it’s worth mentioning now that uh people who
5:00 are pattern vision blind so people who cannot see but have eyes many of them still have these cells these melanops and intrinsically photosensitive cells and can uh essentially match or entrain as we say onto the light dark cycle. >> In fact they possibly have no problems in circadian photo entrainment. They’ll have enormous sleepwake cycle >> but they’re totally blind >> but they are totally image blind. And what’s really interesting is that and this story I heard from Chuck Seizler so I’ll give him credit that some of these
5:30 people who are image blind usually they get dry eyes and they give them a lot of pain and doctors used to think oh then 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 in training to the light dark cycle and are having cyclical jet lags when their clock shifts through the light dark cycle. >> That’s really interesting. And uh I hear
6:00 from a number of blind people, a lot of them have issues 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 match their their schedule. >> Absolutely. >> What is the proper way to interact with light in the first part of the day? >> Honestly, I think the easiest thing is waking up. Get as much light as you can >> into your eyes. >> Yeah. It’s really nice. Your system is primed. If you’re in trained, it’s primed to get light. The sun should be out. Even when it’s cloudy, you’re going
6:30 to get enough intensity to help you adjust your cycle to the dayight cycle. >> These are general rules of thumb, but how long do you recommend people go outside? >> So, if you do it daily, I would say 15 minutes. If you don’t do it daily, you may want to increase it. You do it more, it doesn’t hurt. 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. is going to be perfect. >> Okay. And if um for some reason one finds themsself very far north and it’s very very dense uh cloud cover, how long
7:00 and at what point should somebody consider using an artificial light source to mimic the sunlight? 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 a 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
7:30 independent of the setting of the circadium pacemaker. >> Okay. 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 >> but yeah I mean if you’re honestly 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. Okay. So, I think we’ve um have nailed down that
8:00 first part of the day. Basically, it’s get 10 to 30 minutes depending on how bright it is and try and do that as as often as possible to give the system a regular >> 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 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, that’s a great recommendation. Mhm. >> So, you might want to compensate with some extra time if you missed a day or two. I’ve heard you say before it’s entirely possible to get severely
8:30 jetlagged without traveling. >> Simply by staying in, being on your phone too much, not getting the sunlight. >> And you saw this during the pandemic. A lot of people mentioned that their sleepwake cycles 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 till late at night, your body is going to shift and now your day is going to start instead of like really when the sun comes up, let’s say
9:00 at 6:00 in the morning, it’s going to 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 o’clock at night because now that’s really for for your body is completely different timing. There is this idea of chronotypes that we all each intrinsically have a a best rhythm of either being a morning person. You called yourself an early person or a night owl or more of a kind of standard,
9:30 you know, to bed around 10:30, up around 7 type um thing. And the 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. >> So there is great data on this and there’s a couple of things that complicate this. The first is the people who usually are late, >> what what do you mean? People that wake up late and go to sleep, >> go to sleep late and wake up late, they they have an overwhelmingly higher level
10:00 of depression because human notice that people who go to sleep early and wake up early, they do better in life. They notice that >> they just perform better. >> 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. >> We discriminate against late risers. we in a way we discriminate 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
10:30 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 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 >> right >> and so you you’re just going to be late >> it seems to me is the case is that the only way to really know if you’re meant
11:00 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. So, what should people do in the afternoon/ing time in terms of their light viewing behavior? 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
11:30 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 circadium clock. >> You do keep your home quite dim to dark at night. >> I am an extreme, but I measured it for myself and I asked Reie, my wife, if she’s okay with it. She also liked the dimness. Both of us can see well in in dim conditions. 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
12:00 can. I I call it the minimum amount of light you require to see comfortably. You know, use red light that is very dim if you want to keep the room for sleeping. Red light that is very dim has very small effect on on on circadium clock. and below 10 locks of red light literally doesn’t affect sleep at all. So, there are ways to do it. >> I’ve seen you check your phone after dark >> once or twice. Um, and you did it by
12:30 sort of pointing your phone away from you, right? It actually makes sense that, you know, if you shine a flashlight in your eye, it’s much brighter than if you shine direct line. So, if you just look on the side, most of the light is going to go this way and you’re only missing. And and even when I check sometimes, I check it so fast and switch it off so fast. So ideally I should not check iPhones and iPads. I don’t use iPad at night because it’s hard to lower it enough cuz it’s a huge. But even my iPhone, I try not to use it at night. You had a what I consider
13:00 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. 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
13:30 or even depressed, stressed, learning, etc. >> Bingo. Even if we’re sleeping and waking up at the appropriate times, >> 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 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
14:00 light environment that showed 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 IPRGCS to adjust your circadian clock is not the area that receives the light input for mood regulation. It’s a completely different brain region. And what’s really amazing, this region also receives direct input from the IPRGC’s but projects to areas in the brain that
14:30 are known to regulate mood including the vententral medial prefrontal preffrontal 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 the prefrontal cortex is your executive brain one of the most elaborated in humans to see that they receive input from these ancient photo
15:00 receptor was stunning to us. >> How does that finding inform daily protocols for you or for other people? So that’s why we came up with the tripartate model because as a circadium 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 fun they thought only of the environmental input. But now if you put
15:30 them all together you get with this tripartate model where it’s really mindboggling 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 sleepwake cycle, right? So we know there is a homeostatic drive to affect sleep. We’ve had beautiful talks about that >> which is basically the longer you’re awake, the more you want to be asleep. >> So that’s your homeostatic drive. We’ve talked about the circadian influence of
16:00 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 homeostatic drive if you’re getting light at the wrong time of the day or if you’re being stressed and thinking at the then your sleep is going to suffer. So you have to think of the three together to have a beautiful
16:30 sleepwake cycle. Let’s talk about food and eating and appetite. You had yet another Yes, I I greatly admire your your success in in this way. Yet another incredible discovery showing that there are directs of direct excuse me effects of light on appetite and feeding behavior. So for somebody who’s interested in affecting their eating behavior, how should they use light in
17:00 order to adjust their eating behavior? Right? So now that I’ve told you about all these 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
17:30 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. So regular meal times, >> regular meantimes that fit your circadian clock. So and in fact if you do that when 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
18:00 hungry. Let’s say let’s say you eat at noon. You will not feel any hunger at 11:45 and then all of a sudden the hunger jumps. This is clearly not an energy issue because it could not be that drastic, >> right? No, the desire to eat is mainly driven by these uh these cues, these hormone cues that are very exquisitly timed to exactly >> sleepwake cycle, but also to light. >> Exactly. >> How regular are you or do you recommend people be about meal times? Are we
18:30 talking about down to the minute? Like if I absolutely >> All right. Plus or so 12 noon is my normal lunch. Let’s say uh plus or minus >> half an hour. >> Okay. Yeah. So eat around between 11:30 and 12:30 >> if if that’s the time. And it depends if you also do multiple meals. Remember three meals that’s a decision that somebody came up with. I don’t know why >> nowadays people are fewer people are doing that I think >> given our friend Sachin Panda’s work >> right. I mean, so you could have two meals, you could have very multiples
19:00 meals that are distributed across your active time. >> Some people are not hungry early in the day. They might be late shifted people, in which case eating later in the day will will work well for them. >> Work as long as they don’t eat early in the morning. That’s just you have to work with your schedule, with your active schedule. >> What we’re talking about really is finding your ideal sleep schedule. >> And finding your ideal eating schedule. >> Exactly. and understanding how those two things interact. >> And you know the nice thing as you said
19:30 finding them out is going to help you to understand how they interact because we know from the tripartate model that they are all interconnected and for each person they’re going to be interconnected differently. It’s striking to me that in all animals besides humans, if they deviate too much from the appropriate exposure to light and light dark cycle, they essentially don’t mate andor die andor get killed off. But in humans, we are able to override that at least to some extent.
20:00 But the ways in which we suffer appear to be things like obesity, metabolic syndromes, uh reproductive syndromes that are accompany the other syndromes, you know, endocrine syndromes and mood and depressive disorders. Is there any effort at the level of the the nationally or or laboratories that you’re aware of to try and use light in order to improve mood and mental health? >> I mean, honestly, this is my moonshot. This is the thing that I think people
20:30 because it’s I I I say don’t take a pill take a photo and not I mean you take pills it’s important I’m just making it that really we have an opportunity right now with the incredible advances of LED lights of changing spectra of light of regulating intensities and just to assoc just for simple changes you could really improve sleepwake cycle productivity and still you could actually get more done because
21:00 as we’ve talked about when you have all these messed up now you have to sleep more but your sleep is fragmented it’s not very good >> and you can’t focus >> and you can’t focus when you don’t have alertness when you need the alertness so having all these you could allow you to do even more actually at the end than less and that’s the the exciting part of it >> let’s talk about jet lag >> what are the two or three things that people can do to adjust their schedule quickly. Like let’s say
21:30 fall classes are starting, you start a new job or you have a baby or a puppy or whatever. What is the best way to shift the clock quickly? So, it’s very simple as we’ve talked yesterday. So, imagine you’re in in the outside with no environmental with no industrial light. If you if your body thinks you’re in early evening and you see a bright light, what does this tell you? Oh, wait. This is not early evening yet. It’s still early afternoon or late afternoon. So I have to delay my clock
22:00 to go back to late afternoon. So if you get light early in the evening, it delays your clock. So what does >> meaning that makes you want to go to sleep later? >> Yes. It delays your clock. So later in the night, later in your night and actually it just happens that in humans you get a temperature in a deer later in the night low temperature in your body. After that, lights start advancing your clock. If I understand correctly, what you’re saying is if your typical wake up
22:30 time is say 7:00 a.m., then your low point in temperature probably occurs somewhere around 5:00 a.m. >> Yeah. >> And if you view light right around then, it’s going to essentially advance your >> Yeah. Because then your your body thinks, oh, it’s 7:00, so advance your clock by 1 to two hours. But if I were to view light, say at 3:00 a.m., then it would probably delay my clock. >> Okay. Yeah. So, and then let’s say I land in a new schedule. I want to adjust
23:00 to a new schedule. Let’s say I didn’t manage to do anything with my light viewing before I went and I didn’t I didn’t anticipate the trip. Suddenly, I’m on a new schedule. Okay. I was told that one of the ways to help shift the clock and to avoid gastrointestinal issues is to eat on the local schedule. Mhm. >> to start basically behaving like a local >> even though your circadian clock will take a little bit of time to catch up. >> Absolutely. But you have to remember the light, right? So let’s let’s now that we explained it very simply. Let’s take a
23:30 very simple example, right? New York to Italy. That’s a simple example. New York time, Italy time six hour difference, right? So let’s say you fly from New York at night. You reach Italy at 8:00 in the morning. What is the time in your New York time? You although you reach >> 6 hours back >> 6 hours. It’s 2:00 a.m. >> So when you land Italy, you want to avoid light like the plague. Yeah, you could eat, but you really don’t want to get a light, >> right? Cuz otherwise it’s going to delay. >> It’s going to delay you. It’s going to send you to California instead of
24:00 sending you to Italy, >> right? What Sam is saying is so crucial. Just because getting bright light in your eyes early in the day is really beneficial when you’re at home. When you travel to a new time zone, you have to take into account where your body thinks you are. And so if you’re looking at the Italian sunrise, having just flown from New York to Italy, and you didn’t prepare for that trip by waking up a little bit earlier in anticipation, >> multiple days, >> and you view light at 2, excuse me, at at 6:00 or 7 a.m. Italian time, beautiful Italian sunrise, you are going
24:30 to delay your clock, you’re going to basically throw yourself back to California, but you are in Italy. You’re going to throw your biology back to California and you are going to be up in the middle of the Italian night and you’re going to be ais miserable. >> It’s very important to avoid getting the wrong light information when you’re trying to adjust your body because otherwise it shifts you to the other to the other side. Absolutely right. You are one of these people that has such vigor. I think a lot of your ability to work hard and focus and really do so
25:00 many things at an impressive level is because you think about these issues and you you think about when you’re going to be optimal for focus, when you’re going to be optimal for exercise, when and the when is the key. It’s and I think a lot of people live in the landscape of feeling like there’s something broken inside them. I really agree with you that I think part of the reason I’m continuing to be able to do this that I really think about it and I make sure that I keep everything aligned and and that actually helps me a lot like I
25:30 don’t suffer in sleep I don’t suffer in waking up I never use a timer to wake up system is so aligned it works a lot of times people will say how come I go to sleep I fall asleep fine but then I wake up at 3:00 or 4 in the morning and can’t fall back asleep is it possible that those people were supposed to go to bed at 8:00 p.m. >> Yeah, I mean it is possible uh or it’s possible that their clock is completely misaligned that they are getting maybe enough time at night when they are supposed and then they possibly feel so
26:00 sleepy in the day. So there all these are possible combination. >> Oh that’s an interesting idea I hadn’t considered. So that what they think is their sleep their body is so out of whack with the light dark cycle that it’s actually a nap >> or or the weaker part of the sleep. I mean, you see this in in when you travel to different time zone before you adjust. You go to sleep really well, but 2 hours later you’re fully up. 2 hours. If you were so tired and this is your regular sleep, there’s no way you’re going to wake up in 2 hours. >> Let’s talk about seasonality a little
26:30 bit. Are there other effects of seasonality on humans that we are aware of? >> Honestly, you could see it perfectly, I think, in in Scandinavia. I mean, you could talk to people in who live in >> Sure. They get seasonal depression. >> What? Seasonal depression is one. But actually when you start asking them questions, they tell you like in the winter they barely could wake up. They barely have the energy. Before even depression, even people who don’t get seasonal depression, they’ll tell you our energy level is lower. Our ability
27:00 to go to work is is not the same. And in the summer, most people actually sleep very little. They tell you we we really can we we feel like we’re manic. We have all this energy. And not in a negative way, in a funny way, right? I mean, but if you want to sleep, we have to put these curtain. I think in these situations, you could really appreciate the seasonality of a humans. I think we kind of destroyed our seasonality because we don’t get exposed to that
27:30 much natural light. We have all this artificial light. But I think honestly one of the thing that is going to happen if they follow your recommendation, this is going to cause them to also experience some changes across the season because now they going to see the sun differently. If you’re going to go out in the morning in the summer, you’re going to get a much bright. That’s why I don’t like the change in time. I know people think, “Oh, because you’re biased. You because I think >> Wait, wait, wait, wait. Sorry. The change are you talking about daylight savings?” >> Daylight saving. such a bad idea because
28:00 it disrupts that rhythm that you’re having because I think your body if you keep that rhythm you will see the whole seasonality and I look at it from a different aspect than other people. If you think about it Andrew there is a situation where you’re getting light perfectly well and then all of a sudden they delay it by 1 hour because and then even though it’s the summer your body now if you’re still not adjusting think oh wait what happened? What kind of happened? Well, I’m glad you’re bringing
28:30 this up because I always thought, you know, what’s the big deal? One hour, right? One hour shift, you know, spring forward, fall back, >> adjust one hour. >> But the but this goes back to the beginning of our discussion. It’s not just one hour, >> right? >> Because it’s one hour across that one day, but there’s this cumulative effect on the clock and these uh three elements of your tripartite model, right? The homeostatic, sleep, and the light direct effects on mood. And when it’s so close, it’s sometimes hard to figure out how to
29:00 adjust it perfectly because, you know, we’re already sleepd deprived in our society. And then you shift it by, you know, so it it just it all accumulates and it has no benefit. I just don’t understand why they do this. It makes no sense. Well, I think that the reason they do it is because they don’t understand the biology because one hour seems trivial unless you understand the the repercussions of that 1 hour shift. Because >> what’s also clear now based on what you’re saying is that that 1 hour shift
29:30 is taking you out of alignment with the natural light dark cycle in exactly the wrong direction. It’s pushing people to get even later. >> In the summer when light is going to push you later anyway. It’s really compounding the problem that already exists. >> Samur, this has been um an amazing march through the importance of light. I’m certain that people are going to start thinking about how to change their relationship with light as a way to anchor everything that they do and
30:00 that’s important to their health. Let’s talk a little bit about where people can find you. Your laboratories at the National Institutes of Mental Health. He is head of the chronobiology unit. all these things as I’ve mentioned earlier but um you are active on Twitter and Instagram >> so what is your uh Twitter handle >> it’s Samarhat and I think the same for >> definitely give him a follow there and on and on Twitter and um and I’m sure that he’ll be happy to answer questions uh and enter entertain any and all
30:30 discussions about chronobiology >> absolutely yeah and light yeah >> great thank you Sam >> awesome thank Thank you. [Music]