Making Sense with Sam Harris - #267 — The Kingdom of Sleep

Episode Date: November 10, 2021

Sam Harris speaks with Matthew Walker about the nature and importance of sleep. They discuss sleep and consciousness, the stages of sleep, sleep regularity, light and temperature, the evolutionary origins of sleep, reducing sleep, the connection between poor sleep and all-cause mortality (as well as Alzheimer’s disease, diabetes, obesity, and heart disease), sleep across species, learning and memory, mental health, dreams as therapy, lucid dreaming, heart-rate variability, REM-sleep behavior disorder and parasomnias, meditation and sleep, sleep hygiene, different types of insomnia, caffeine and alcohol, sleep efficiency, bedtime restriction, cognitive-behavioral therapy, napping, sleep tracking, and other topics. If the Making Sense podcast logo in your player is BLACK, you can SUBSCRIBE to gain access to all full-length episodes at samharris.org/subscribe.   Learning how to train your mind is the single greatest investment you can make in life. That’s why Sam Harris created the Waking Up app. From rational mindfulness practice to lessons on some of life’s most important topics, join Sam as he demystifies the practice of meditation and explores the theory behind it.

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Today I'm speaking with Matthew Walker. Matt is a professor of neuroscience and psychology at UC Berkeley and the director of its Sleep and Neuroimaging Lab, and he's also a former professor of psychiatry at Harvard University. He has published over 100 scientific studies and has appeared on 60 Minutes, Nova, BBC News, and many other outlets. His first book, Why We Sleep, has been an international bestseller. And he also hosts his own podcast, the Matt Walker Podcast. I've been wanting to speak to Matt for quite some time because, as you'll hear, I've been increasingly worried about the quality of my own sleep.

I'm late to the party here, but now I'm convinced of the importance of sleeping well most nights. And Matt and I get into all the details here about the nature and importance of sleep. We discuss sleep and consciousness, the stages of sleep, sleep regularity, light and temperature, the evolutionary origins of sleep, the generally doomed attempt to reduce one's need for sleep, the connection between deficiencies in sleep and all-cause mortality, Alzheimer's disease, diabetes, obesity, and heart disease, the role that sleep plays in learning and memory, and mental health, heart rate variability, REM sleep behavior disorder, and various parasomnias. We discuss lucid dreaming, dreams as a kind of therapy, the connection between meditation and sleep, the various forms of

insomnia, and there are practical tips for what to do about them strewn throughout our conversation. We discuss sleep hygiene, caffeine and alcohol, sleep efficiency, bedtime restriction, napping, and finally sleep tracking. And as we're here on that final topic of sleep tracking, Matt and I discover that each of us is associated with the company Aura that makes a sleep tracking ring. I am a minor investor in the company, and Matt is its scientific advisor. Neither of us knew about the connection before we started talking, and you'll hear I have a bit of a love-hate relationship with my own aura ring. It is a remarkable device, but I may have what Matt calls orthosomnia, which is an overabundance of concern about my sleep data.

In any case, make of that what you will, and I hope you find this conversation useful, as it runs nearly four hours. And now I bring you Matthew Walker. I am here with Matthew Walker. Matt, thanks for joining me. It's a delight and a privilege to be speaking with you, Sam. Thanks for having me. So you've written a book, Why We Sleep, that seems to have gotten

into the hands, if not the brains, of more or less everyone. And now you have your own podcast, the Matt Walker Podcast. And you have been on many, many podcasts that I've noticed, talking about the science of sleep and seemingly almost single-handedly making people newly aware of the importance of sleep in their lives, both from the side of physical health and mental health, emotional regulation, really just across the board when you're talking about human well-being, the difference between good and bad sleep seems paramount. And I must say, I have really neglected sleep as a variable for most of my life. In fact, I think I was early in life, toyed with the fairly crazy ideal of limiting sleep so as to boost productivity. And we'll get into all of that. But before we dive into the specific chapters of our conversation

here, perhaps you can introduce yourself, your background intellectually and academically, and just tell us how you came to focus on sleep. I wish I could take the compliment of bringing sleep back onto the public awareness map. I stand on the shoulders of many of my colleagues and they are astronomically wonderful, so I try to do my part. In terms of my background, I am a professor of neuroscience and psychology at the University of California, Berkeley in America. And I've really tried to dedicate myself to understanding the question of why we sleep for the past 20 years. I think like most people, I am an accidental sleep researcher. I often think, you know, when kids are young and the

teacher says, tell me what you would like to be when you grow up, no one's shooting their hand up in the classroom and saying, I desperately want to be a sleep researcher. Yeah. And I can attest that when I started my neuroscience PhD, someone from a sleep lab, I forget who, tried to recruit me to their lab. And I thought, why would I want to study sleep? I had no interest at that point. And now I feel some chagrin over that dismissal because it is increasingly fascinating and, as I said, consequential. And in some ways, I don't blame you. Maybe at the time, certainly even 20 years ago, one could argue it's almost academic suicide to suggest that you want to become a sleep researcher. And not necessarily truthful, but some would argue that it was almost a charlatan science

to begin with. And of course it it is it's the most bizarre strange illogical irrational from an evolutionary perspective idiotic thing that an organism can do and you're going to leverage an entire academic career on that platform good luck and good night would be the i think the tagline but i was studying for my PhD, people with different forms of dementia. And I was using brainwave patterns to try and differentially diagnose them very early on in the course of, of dementia. And I was failing miserably, couldn't get any good results. And one weekend I had this little igloo of journals that I would retreat to, which tells you everything about my social life. And I started to learn that some of those dementias would eat away at sleep

centers and other forms of the dementias would not, because there are many different forms of dementia. So I realized I was measuring my patients at the wrong time, which was when they were awake, and I should be measuring them when they were asleep. I started doing that. I got some fantastic results. And at that point, I started to ask the question, I wonder if these sleep disruptions and impairments are not a consequence of the dementia. They're not a symptom of the dementia. Maybe they are a cause of the dementia. But I realized 20 years ago, no one could answer a very fundamental question, which was, why do we sleep? And I think the crass answer at that time was that we sleep to cure sleepiness, which is the fatuous equivalent of saying, I eat to cure

hunger. It tells you nothing about the unique benefits but then I started to explore this thing called sleep and I fell absolutely in love with it and to this day 20 years on I still think it is the most beguiling thing in science it is a love affair that's not left me for all of those decades. And I remain an amorous partner to its wonderful gifts, both nightly as a practice and also from an intellectual and academic and research perspective. Does that give some background? Yeah, yeah. If I can follow your romantic analogy here, sleep is a fairly coy mistress for many of us. Speaking personally, this has always been not even on the back burner for me as a problem to solve in my life. I've accustomed myself to sleeping badly and just accepting on some level that I sleep badly. And so,

encountering your work is fairly arresting to someone in my condition, because the stakes, as we will elucidate here, are incredibly high, given the connection between sleep and health. So, I wanted to, at the outset, address the component of worry here, worry about sleep, because many people listening to us will also recognize in themselves that their sleep is far from ideal, and to add a layer of worry to that is obviously counterproductive when the goal is to make it easier to sleep soundly and on some better schedule in general. So can you address this effect that our conversation is likely to have, especially when we're talking about possible links between poor sleep and dementia and all the rest? It's very easy to begin to treat this

as some kind of medical emergency in the offing. What do you have to say by way of guidance or caution on that point? In some ways, it's a rock and a hard place that I found myself in. This is something that I've learned since publishing the book. And I think it's something that I've corrected in my communication to the public. As I was writing the book at the time, at least within the public sphere, as you mentioned, sleep was the neglected step system in the health conversation of today. And it was that way. And I was so familiar, as all of my colleagues were, with the disease and the sickness and the suffering that was happening because of this sleep deficiency that was so

pernicious throughout most first world nations, that I wanted to try to, no pun intended for either this podcast or the topic, but sort of wake people up to the fact of the importance of sleep. And I think that in my communications and maybe even in segments of the book, I was perhaps heavy handed and I had neglected to recognize the concern for the sleep anxious and those who are having sleep difficulty. And I've since become so much more sensitive to that. And I can't deny the science. I can't not tell you about the links between insufficient sleep and, you know, Alzheimer's disease, obesity, diabetes, cardiovascular disease, depression, anxiety, cardiovascular disease, depression,

anxiety, even suicide, some forms of cancer. But I also don't want people to become overly anxious, but how do you do that? How do you find that sweet spot? And so for me, it's been a real lesson, and a lesson also because I am no poster child for sleep. I have had my battles and I did not mention them in the book. And I think I should have. I'm being personally open. I'm a very private person. I've had at least three bouts of insomnia during my lifetime and they were vicious.

And just because you know a little about sleep doesn't mean as though you are immune to its vagaries. It is a mistress that can be very fickle. So I think for this podcast, it's important to keep in mind two things. First, everyone has a bad night of sleep. And if you're there at night struggling to fall asleep, don't worry. Even with all of the facts and the science that we will discuss, it's not the worst thing in the world. The second thing is that if you are persistently and continuously chronically struggling to sleep, you don't have to.

Because there are efficacious treatments, many of them non-pharmacological, which is great, that can help course correct. In fact, even in older adults where you think there is no hope at all for a solid night of sleep, those therapies, many of them seem to be beneficial to restoring some degree of good sleep. So you don't have to suffer in the nighttime silence that there is benefit there. I think that that's perhaps the best way to approach it with sensitivity, compassion, understanding, but truthfulness to the science. You know, I wouldn't want to make people nervous about, you know, eating so precisely that it doesn't change their blood sugar set them on a path towards you know pre-diabetes or type 2 diabetes and where you become so obsessive and anxious that

food and the joy and pleasures of eating start to fail i also don't want to do that with sleep but i equally don't want to tell you that it's fine just to eat a pint of ice cream every night and that your blood sugar won't suffer. I'll tell you about that science too. Yeah, yeah, well, so that's great by way of introduction, and we will get into all of the aspects here, including all of the practical recommendations you have for improving sleep and bypassing any perverse cul-de-sac of worry about sleep that can get in the way of that project. So let's just begin.

Let's jump into our first chapter here on what sleep is, even before answering the question that is the title of your book about why we sleep. What is sleep? From a functional perspective, I think the headline statement you could argue is that sleep, physiologically at least, is perhaps the single most effective thing that we can do every day to reset the health of our brain and our body. And that's not to dismiss food or nutrition or exercise. But if you were to take you, Sam Harris, and I were to deprive you of food for 24 hours, deprive you of water for 24 hours, deprive you of physical activity for 24 hours,

or deprive you of sleep for 24 hours, and I were to look across your brain and your body and see which one demonstrates the more demonstrable impairment, By a very large margin, it's sleep. But I don't want to sort of do that Coke, Pepsi, Dr. Pepper, I'm still missing one, I can't think of it, challenge. So you could ask from a functional perspective what sleep is. You can also ask what is sleep as a process that unfolds across the night in terms of its architecture? And then you can also ask and debate what is sleep as a conscious state versus a non-conscious state? And so I'm happy to maybe speak about how sleep unfolds, since that may be the logical entry point, or just go straight into how we can noodle and wrestle with the idea of it being a

conscious versus non-conscious state, which can get us into tautological waters. But you tell me which of those two perhaps would be best to start with or fruitful for you. Yeah, well, the question of whether it's conscious is, and I know I've spoken about this elsewhere, is very difficult to resolve just because it's difficult to discriminate an interruption in consciousness from a mere failure of memory. So for instance, dreams are routinely conscious, but it's also possible to have dreams and not recall them at all. And then one could wonder whether those dreams, you know, whether those stages of REM sleep were actually associated with conscious dreaming. And one could wonder that the state of deep sleep is also a state of conscious enjoyment

of something quite formless and profound, but there's just no memory of it. And so we read it as just a loss of experience for that period. And so I don't know how we would, I mean, I'm happy to hear anything you think on that topic, but I'm unaware of anything that would resolve that for us. I think it's a very elegant point, which is we rely for that question in part subjectively from the sleeper themselves, a report of whether or not they were experiencing anything going through their mind just before we woke them up and said, you know, what were you having as an experience? And that suffers from the failures of memory, which we know happen. Just because you don't remember your dreams doesn't mean that you weren't dreaming. I think one way that you can get closer, but we will still fail, is to split that question apart on the basis of perception.

Which is to say, depending on your... I mean, behaviorally, the way that we define sleep in other species where we can't, for example, stick electrodes on them, the way that we define sleep in other species where we can't, for example, stick electrodes on them, is as a condition in which the organism stops responding to the outside world, which is about perception. Does this mean that we are not conscious during sleep because we typically stop responding to the outside world in all stages of sleep? And that depends on your definition of consciousness, but we stop interacting with, and for the most part, perceiving the outside world, which some would argue I can have electrodes on your head and I can play sounds while you're asleep that don't wake you up. And I can still see that the brain at some level is processing those sounds in a way that is not dissimilar to the way it does when we're awake,

consciously perceiving those sounds. We can do fMRI studies and we can play those sounds as you're sleeping in the MRI scanner. It's hard to believe that people can, but they do sleep in the scanner. And you can see that there are different ways of perception. There's a great study that looked at new mothers and what they found was that when they played the cry of that infant versus another sound, even though they remained asleep, it was a very different network, a salience network activated in response to the child of that mother versus another sound of equal volume, etc. So there's definitely some degree of processing and discriminatory

processing, but I still don't think it's the same non-conscious state as anesthesia, meaning that there is still some degree of perception of the outside world during deep sleep. In other words, what we call extraception, the ability to focus or sense the outside world. Well, there's got to be just based on the fact that you can wake somebody up from deep sleep. So that's got to get in somehow. That's exactly it. Yeah, I think you exactly predicted where that conversation was going, which is that no matter what stage you're in,

sleep at least is a condition in which it is environmentally reversible. For example, if a sound is loud enough or if someone were to pinch your skin hard enough, which would be a desperately cruel thing wouldn't it to do when someone's asleep you would wake up from sleep which is to say that in sleep we are unresponsive but that state of unresponsivity is reversible now that's not true of anesthesia or death for as best we can tell so i think it's very hard to argue then that we don't have a very

substantive yet qualitatively different form of consciousness when we dream, especially during when we go into REM sleep dreaming. So I think we can get a little bit closer to a dissection of what do we think of as the state of conscious processing during sleep. But I still feel as though I don't see data that can really solidly give us one argument in either favor, conscious, non-conscious state. Yeah, I would just add here that conversely, there are states of meditation or drug intoxication where someone is also totally unresponsive to the outside world, but all too conscious of something, right? I mean, in terms of their subjective report once they come back from those experiences. So there's kind of a double dissociation here. So I think

responsiveness to stimuli isn't the cut we need. We obviously need the neural correlate of consciousness where we can just scan your brain and say, you know, by some methodology and say, okay, this is the footprint of consciousness in the human brain, and it winks out in this condition, let's say general anesthesia, and it's attenuated to this degree in this stage of sleep. But unfortunately, we don't have that yet. And I think there are conceptual and operational limits to our getting it. Again, the role of self-report is always potentially confounding and seditious here, because you can just, you know, we just need a sufficient cohort of people who are reporting things that occurred in the chapter that we're deeming to be unconscious. And either we're going to think they're delusional or they're lying or they're in some other way wrong, or that's going to erode our confidence that really the lights are out during that epic.

I think self-report, speaking about fickle mistresses, is so prone to all of those errors. Okay, so with that caveat in mind, let's launch into... It would be good to just give us the structure of sleep here in human beings. You can say anything else you want about other animals, but what is sleep for people? Sleep, at least in human beings, and in fact, in all mammalian species, as long as they are land-dwelling, there's a caveat there too, is broadly separated into two main types. On the one hand, we have non-rapid eye movement sleep, or non-REM sleep for short. And on the other hand, we have rapid eye movement sleep, or REM sleep. I often want to make people clear on the fact that that's named not after the popular 1990s Michael Stipe

pop band, but because of these bizarre horizontal shuttling movements that occur during the stage of sleep. That's where it gets its definitional name from. And coming back to non-REM sleep, which I always feel sorry for, by the way, isn't it sad to be defined by something that you're not? You are not REM sleep. I guess in this case, you're deep and light. That's correct. So non-REM sleep is then further subdivided into four separate stages, increasing in their depth of sleep. So stages one and two are what we would consider, or your sleep tracker will probably try to tell you, are the light stages of non-REM sleep, whereas stages three and four, that's the really deep non-REM sleep. And REM sleep then is the stage in which

we principally dream. Depending on your definition, dreaming isn't exclusive to REM sleep, but for what most people would say in the lay public, this is dreaming, what they're really referring to are the bizarre, narrative, hallucinogenic, emotional, memory-laden experiences that come from this thing called REM sleep. So those two types of sleep, non-REM and REM, will play out effectively in a battle for brain domination throughout the night. And that cerebral war between non-REM and REM, in humans at least, and it's different for different species, will last about 90 minutes. And that creates, for the average adult, a prototypical 90-minute cycle where you go into non-REM sleep and then you go into REM sleep. But what changes, however, is the ratio of non-REM to REM within

those 90-minute cycles as you move across the night. So in other words, in the first half of the night, the majority of those 90 minute cycles are going to be comprised of lots of non-REM sleep, particularly deep non-REM sleep. But as you push through to the second half of the night, that sort of seesaw balance shifts over and those 90 minute cycles are comprised of much more rapid eye movement sleep and very little deep sleep. And that has some consequences that we can also talk about. But I would probably mention also every one of those stages of sleep, or almost all of those stages of sleep, we have now learned are important. There is no one more important stage of sleep than the other. Now,

you can argue, well, what are you talking about importance? You're talking about mortality risk and death, and we can use that as a filter to debate that as well. But overall, different stages of sleep provide different functions for the brain and the body at different times of night. So we need all of those stages. And is it true that we generally wake up however briefly and indiscernible after each of these 90 minute phases? You get through your REM period and then there's a brief awakening? That's absolutely, you definitely need to be a sleep researcher. Take a sabbatical and- Build me a time machine and I'll go back and have the conversation differently.

So we do know that usually at the end of every one of those 90-minute sleep cycles, at the end of each of those REM phases, there is a brief termination of sleep where we wake up. And in part, we think that that's perhaps because of the need to maneuver the body and change the body's position. And so we have these brief awakenings. They're usually so brief that most of us don't recall them. They're not imprinted in memory, but everyone will typically have a brief awakening and then a movement episode after where they shift position. Right. And we'll talk about sleep tracking and the tools that are available to do that personally beyond going into a sleep lab and getting totally hooked up. But viewing these stages

in their totality, you've said that each is indispensable, but it does seem, at least in the way one communicates the imperative to get all of these stages, most of us are not deficient in the stages of light sleep. And it's really the stages of REM and deep sleep that are marketed as truly restorative, right? And those are the areas of real deficiency. I mean, so for instance, if someone was sleeping six hours, but they got very long epochs of deep sleep and REM sleep, would that strike you as a much healthier profile than someone sleeping six hours,

but it's mostly devoted to the stages one and two of light sleep? Yes, I think that that's fair to say. We do need stage two as well. We've discovered that stage two non-REM sleep is associated with certain forms of memory and memory processing. And there is a particular electrical feature of stage two non-REM sleep, which continues on into deep non-REM sleep stages three and four called sleep spindles, which are these beautiful little champagne cork synchronous bursts of electrical activity that happen during stage two non-REM sleep and then stages three and four. They last for about a second and a second and a half, and they seem to be critical for a number of different processes of both the brain and they seem to transact or be at least associated

with several benefits for the body. But overall, I would say that it's very difficult to have a night where you're not transitioning because when you go down into deep non-REM sleep, you have to progress through stage two. And when you're coming out of deep non-REM sleep, you have to progress through stage two non-REM sleep, again, the lighter form of non-REM sleep, before you get up into REM sleep. And so it would probably be rather difficult. You can

manipulate conditions in which this can happen, which I won't bore you with, but where you could have the scenario that you described, but for the most part, you're still going to get that stage two non-REM sleep. Yet what you said is correct. Well, this is where I'll seed you with practical questions throughout, but the first that comes to mind here is what are the implications of waking with an alarm clock versus waking with the change in lighting conditions born of sunlight coming through the window? I guess there's the implication of using a sleep mask or blackout curtains where you're not getting those environmental light cues. I can imagine, you know, if you're unlucky, your alarm clock rings when you're in stage four sleep, say,

and you're brought out of that in a less than ideal way, what are those effects and what do you actually recommend if a person's schedule allows for it? What do you recommend as a mode of waking up in the morning? Unless you are waking up within the first couple of hours of sleep, it's unlikely that your alarm would wake you up in the deep stages of non-REM sleep. That's not true, however, if you take an afternoon nap and that nap lasts a little bit too long. And by too long, what I mean is you're going past that sort of 20 to 25 minutes and you're starting to go down into the deep sleep. And then your alarm wakes you up, then you almost have this kind of sleep hangover for the next hour or so.

Those naps are terrible. Yeah, with a change of time zone when you have terrible jet lag and you decide, okay, there's no way I'm going to make it to the evening, so I'm going to give myself an hour to sleep here. And waking up from that hour is just about the worst wake up one ever gets. It's pretty grim, isn't it? And it's what we call sleep inertia, where you get a state carryover where your brain

never typically wakes up from, is jolted out of that deep sleep naturalistically from an evolutionary perspective across millions of years, that's not been the case. And so we're not well prepared for recovering from that assault. And therefore we suffer this terrible sleep inertia. So it's not so likely to happen, but when it does happen, it's grim. It can also happen at night when, for example, you get a phone call and all of a sudden it wakes you up at, you know, 2.30 or 1.30 in the morning. And once again, you're jolted out from that deep sleep. And yes, you can answer the phone and you can be somewhat responsive, but it is just

grim. You're in this total treacle haze of cognitive dysfunction. And it's all you can do to allow words to tumble in some meaningful way, one foot in front of the other out of your mouth. So that is perhaps a less likely circumstance. What would I suggest? It's difficult because one of the critical things that people need to do to get their sleep back on track is the simple act of regularity, which is going to bed and waking up at the same time, no matter whether it's the weekday or the weekend. And for that, we often require an alarm clock. And I also advocate for people not just to have an alarm clock in the morning, but why don't we have a to-bed alarm as well as a to-wake alarm? And it's one way to help keep us

on schedule and track. I would say, however, that if you study hunter-gatherer tribes whose way of life hasn't really changed for hundreds if not thousands of years, they don't seem to wake up in an artificial manner. And if you ask them, you know, do you find ways to force yourself to wake up? They find it a perplexing question. Why would you, why would you terminate something that's not yet complete? It's a little bit like saying, why would you go out to your favorite restaurant, order your favorite dish, have two bites of that dish, and then get up and walk out? You would stay until you're full when you are complete with that meal. And why would we wake up when we are not yet full of the sleep that we need? And mother nature will take care of that. When it's time to wake up and we've had the sleep that we need, we do. So one way some people will ask me, how do I know if I'm getting enough sleep? It's not the ideal way, but one suggestion is to say, if your alarm clock didn't go off in the morning, would you sleep past that alarm? And if the answer is yes, then you're still carrying some degree of a sleep need, which means that by waking up artificially, you're inducing a sleep debt as a consequence.

What about the role of light cues in bringing someone out of sleep? We used to think that light perhaps was the trigger of or one of the facilitating functions for rising people out from sleep in the morning. And again, by looking at those hunter-gatherer tribes, what we found is that that's not really the case. They often typically will wake up a little bit before the dawn. What seems to be the trigger for the arrival of wakefulness and the termination of sleep is more so temperature, both the internal temperature and the ambient temperature rising, because often they will sleep with the environment, with the ambient temperature, unlike many of us in modernity where we have a controlled temperature.

So that's not to suggest that light can't be a facilitator to help you wake up in the morning. And in fact, I will, I have one of these little smart lights next to my bedside and I program it to try and say, you know, two minutes before the time that you're supposed to wake up, start to bring light into the room. I would say though that I do have an alarm myself. My alarm is, and we can get into sort of chronotypes and what your preference is, but my alarm is set for around 7.04 in the morning or at 7.04 in the morning. Not because there's anything special or unique, please don't go rushing out and changing your wake up time to that. We're not going to have a chapter on numerology here and the significance of even numbers.

The only reason I do that is why not just be idiosyncratic? Why would you set it at, you know, 7.05 or 7 or 7.10? Just why not 7.04? That tells you probably everything about me and why I'm desperately unpopular. But I usually wake up naturally. I would say about 80% of the time I wake up naturally before my alarm clock. So I think one of the worries that people have when I tell them to do the experiment, if you have the luxury and the schedule flexibility to do it, stop your alarm and just sleep in the way that you are your body wants to sleep the greatest worry is that my goodness I normally wake up at seven and I'll probably wake up at nine o'clock in the morning is the first concern now that may be true to begin with for the first few days

because you're probably trying to sleep back a debt that you've amassed chronically over weeks, if not months or years. And the second problem is that when people sleep long, they wake up and once again, they have that strange sleep hangover effect where if they get nine hours of sleep, they feel worse than when they get seven hours of sleep. That is typically because you are in the phase of paying back the debt. And if you let that experiment play out for another week, you wash away that sort of pressure to sleep. Now, we can speak about sleep debt and whether you can ever truly pay back the bank or not. But that goes away with time. It's sort of like detoxing from a drug.

At first, it's brutal and you have all of these side effects, and you have a withdrawal syndrome. And in some ways, that's the withdrawal syndrome where you start sleeping longer. That settles down. It's like a Richter shot, and then it finds a sweet spot. And gradually, you will actually acquiesce

to your typical sleep need and your sleep profile. Most people don't have the luxury to do that. So light can be helpful. Temperature is one. I also have one of those smart home thermostats. And temperature is critical for sleep. We need to ironically warm up to cool down to fall asleep. And then we need to stay cool to stay asleep. And finally, we need to warm up to wake up. And so you can create a bespoke tailored temperature profile for your night of sleep that can help to some degree. Now, of course, you're under the sheets and the ambient has some role to play, but it's also altered by what's going on locally underneath

the sheets too. So you can't control it exquisitely. And that's where smart mattresses are coming in to try and take that out of the equation. So those are some of the ways that you can play around with sleep. I do like the idea if you are, particularly if you are a night owl and you struggle to wake up at the time that society forces you to, which is not in synchrony with your morningness or eveningness preference. You can use light in the morning, but then you can reverse that trick in the evening where you try to ensconce yourself with as much dim light and darkness to help you try to get to bed a little bit earlier so it's not as though light should be dismissed and you know blocking devices blackout curtains eye masks earplugs sound is another pollution that will disrupt your sleep

i will typically use all of those i have blackoutout curtains, I have an eye mask, and then I have earplugs. I think I'm starting to sound like the Woody Allen neurotic of the sleep world, but that's just me. Yes, well, all we need is one picture of this setup and to completely discredit you as a expert on sleep. Oh, I've been so discredited by lots of different things, but that would, I think, seal the deal. Okay, so let's transition to the question of why we sleep. I think there's probably no real boundary between what sleep is and why we do it conceptually here, at least in places, because part of the story here is the evolutionary question of just why sleep is a thing, how it came to be that animals like

ourselves dedicate so much of their lives to this state that seems fairly pointless and even dangerous. I mean, this is the, you can imagine in civilization, the danger is less salient, but just imagine how precarious it would be to, you know, go out in the woods where there are bears and perhaps several other species that could consider you a meal, and to just take eight hours of darkness to be unconscious for. I guess there's a potential evolutionary answer there in that the one thing you're not doing when you're sleeping is stumbling around in the dark where you're not very good at seeing, and several other things can see you better than you can see them, but I'm not sure that's an adequate rationale. So let's begin talking about the origins of sleep as we know them or can

hypothesize about them. What do you think about why sleep even exists? So far in every species that we've studied to date, sleep or something that looks very much like it seems to exist. And what that is suggest is, is that sleep evolved with life itself on this planet and has fought its way through heroically every step along the evolutionary pathway. Let's linger on that point because that's very interesting because you can imagine the adaptive benefits that would generally accrue to any species that could just get over its need for sleep. I mean, there would have been, you would think, a selective pressure in the direction of completely erasing sleep.

So it suggests that it's rather hard to do. I think it's a beautiful way of thinking about it, because from an evolutionary perspective, just as you noted, it is the most idiotic of all things. Firstly, when you're asleep, you're not eating, you're not foraging for food, you're not finding a mate, you're not reproducing, you're not caring for your young, and worst of all, as you noted, you're vulnerable to predation. So on any one of those grounds, but especially all of them as a collective, sleep should have been strongly selected against during the course of evolution. And it's once been said that if sleep doesn't serve an absolutely vital function, then it's the biggest mistake the evolutionary process ever made. And what we've now since learned is that Mother Nature didn't make a spectacular blunder in creating this thing called sleep but even very old evolutionary you know species like earthworms for example

seem to have periods of it's called lethargicus or essentially a sleep-like state you know this takes sleep back millions of years even some bacteria that seem to live at least several days, they will have an active phase and a passive phase, perhaps the precursor to sleep. So you're right, you could well imagine why if some species had understood a way to circumnavigate its way around the essential need for sleep, it would have dominated for lots of different reasons, at least within its species category. The fact that we haven't seen that yet argues that sleep must be fundamental at the most basic of biological levels. And it's one of the reasons why when people will say to me, well, look, can't you, you know, if you're a doctor training, I think we learned

to overcome our need for sleep. We learned to tolerate and deal with insufficient sleep. And you can do that. If you could, trust me, I think, you know, there's some degree of hubris there, which is Mother Nature, if she could have even halved the amount of time that you are vulnerable to all of those vicissitudes of sleep she certainly would have and the fact that it's been preserved tells you that doesn't seem to be possible and within the lifespan we think that we can come along and within a 10-year training over career we could overcome it it's it's unlikely to be the case reigning over Korea, we could overcome it. It's unlikely to be the case. Actually, we might punctuate this part of the conversation with the cases of various people who,

at least by their own testimony, have gone a fair way toward overcoming their personal need for sleep. I think it was Winston Churchill who, during the war years, was sleeping the last 10 minutes of every hour or something like that. I don't know if that's apocryphal, but what do we know about anyone's successfully titrating their sleep down to something like a minimum? I'm sure there are genotypes here that we may know something about where people just require less sleep than is normal. But actually, I once had a doctor who claimed to sleep no more than three and a half hours a night. And, you know, whether he was, again, this is before the age of sleep tracking, so he could have been delusional. But what do we know about people who sleep much less than you would recommend? Firstly, from an epidemiological or population-based perspective, which is simply

associational, using that sweet spot that we recommend, which is somewhere between seven to nine hours a night for the average adult, once you start to get less than that, the shorter your sleep, the shorter your life. That short sleep predicts all-cause mortality. Are there people in history who have claimed to be short sleepers? There are. And Churchill was one. Edison was another, although Edison was a habitual napper during the day, and he used naps and sleep as a creative tool. day and he used naps and sleep as a creative tool. Then you have Thatcher, Margaret Thatcher, you have Ronald Reagan. Just named two people who ended their lives with Alzheimer's. So that's not a great commercial for their strategy. Yeah. They seemed, on the face of it to make it through until the, you know, 50s or even 60s, my goodness, there is evidential proof that you can sleep what they claimed to be sleeping, which is four hours

a night, and get away with it. And ultimately, what we learned is that one way or another, sleep deficiency seems to get its hucks into you, that the elastic band of sleep deprivation can stretch only so far before it snaps. And tragically for both of those individuals, Thatcher and Reagan, they came to the disease of Alzheimer's. And we now know that there are... I now realize we have several files open, but each of these seems important. So on that point, but each of these seems important. So on that point, how do we disentangle association and causation here? Because couldn't it also be true that one of the early symptoms of Alzheimer's or being at special risk for it is to have one's apparent ability to sleep diminish over the course of one's life, even maybe starting as early as one's 30s or 40s?

Yeah, so we can go, Alzheimer's disease is actually a great example. It's probably been, I think, one of the most exciting areas of sleep research in terms of discoveries in the past 10 or even 5 years. We started with just those epidemiological associations, which are simply that, they're correlation, they're not causation. And what that told us is that people who were reporting sleeping less than 6 hours a day... If you'd like to continue listening to this conversation, you'll need to subscribe at samharris.org.

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