Why We Need Sleep, with Dr. Matthew Walker

In this episode, we discuss:

• How Dr. Walker became a sleep researcher
• What we’ve learned about sleep
• How our sleep patterns have changed
• Tracking your sleep
• Sleep patterns of night owls and morning larks
• How naps, meditation, and medication affect sleep
• Five things you can do to improve your sleep

Show notes:

• Why We Sleep, by Dr. Matthew Walker

Hey everybody, this is Chris Kresser. Welcome to another episode of Revolution Health Radio. This week I’m really excited to welcome Dr. Matthew Walker as a guest on the show. Dr. Walker is an English scientist and professor of neuroscience and psychology at the University of California Berkeley, my alma mater. His research focuses on the impact of sleep in human health and disease. Previously, he was a professor of psychiatry at Harvard Medical School. He’s also the founder and director of the Center for Human Sleep Science.

He has received numerous funding awards from the National Science Foundation and the National Institutes of Health and is a Kavli Fellow of the National Academy of Sciences. He has published more than 100 scientific research studies and has been featured on numerous television and radio outlets, including 60 Minutes, Amanpour & Company, National Geographic, NOVA scienceNOW, The Joe Rogan Experience, NPR, The Peter Attia Drive, and the BBC.

I’m really looking forward to talking with Matt because he is really the foremost expert in the world right now, I think, when it comes to sleep, why we sleep, what sleep does for us, what happens when we don’t sleep, and what we can do to improve our sleep. I’m really looking forward to this conversation, and I hope you enjoy it. Let’s dive in.

Chris Kresser:  Dr. Walker, thank you so much for being here. I’ve really been looking forward to this.

Matthew Walker:  It’s a delight and a pleasure to be here, Chris.

Chris Kresser:  So, I always like to start with a bit of background. Can you tell us how you got interested in studying sleep in the first place?

How Dr. Walker Became a Sleep Researcher

Matthew Walker:  Yeah, I think, I mean, it’s interesting. I don’t think anyone, when they’re growing up, wants to be a sleep researcher. When you think about that classroom of young kids and they say, “What would you like to be when you grow up?” And hands are shooting up, astronauts, doctor. No one says sleep scientist.

Chris Kresser:  Yeah, I can’t say I recall anyone saying that in my classroom.

Matthew Walker:  And they have to change that at some point. But I think, for me, perhaps like most people, I became an accidental sleep researcher. I started off at medical school, which you can do at age 18 back in the United Kingdom. After a couple of years, one of the professors pulled me aside and said, “Look, you’re always asking questions, and you’re not doing that for the wrong reasons. You’re doing it for the right reasons. But doctors perhaps are more interested in answers than they are questions. So I think you actually may be a scientist rather than a doctor.” And, now, that was a time when we didn’t have sort of MD, PhD programs. It was a single track kind of mentality.

So I shifted to neuroscience, brain science. And for my PhD, I was studying brainwave patterns in people who had very early signs of dementia. And we were trying to use those brainwave recordings to differentially diagnose which type of dementia they had, such as vascular dementia, or Alzheimer’s dementia, or Parkinson’s. And I was failing miserably. I was getting no good data. And every sort of weekend, I’d finish up on Friday at the libraries and I’d take all of my medical journals home in my doctor’s residence. I’d have this little igloo in my room of journals that I would sit in over the weekend and read. And one weekend, I realized that these different types of dementias, some of them would eat away at the sleep generating centers, and others did not. And I thought, my goodness, maybe I’m recording the sleep of my patients at the wrong moment in time, which was when they were awake, I should actually be measuring them whilst they’re asleep.

And I got a small grant, set up a sleep laboratory, didn’t know what it was doing for the first couple of months, figured it out. And then that’s where the great results came. At that point, I started to then ask the question: I wonder if sleep is not just a symptom of degenerative dementia. I wonder if it’s a predisposing risk factor or a causal trigger. And that’s when I thought, oh, goodness, so what is this thing called sleep? I realized that no one could answer the simple question back then, which was 20 years ago, which was “Why do we sleep?” And I thought, well, that’s a great question. I’ll go and answer that in a couple of years and come back to this question, not realizing that some of the most astute minds over centuries had failed to crack that question.

And I thought, without any hubris, I was just going to do it in two years. And that was 20 years ago. And I think what I’ve learned is that hard questions care little about who asks them. They meter out their lessons of difficulty all the same. And I have been schooled. So that that was my accidental journey into sleep.

Chris Kresser:  That’s great. What a helpful mentor you had in that process.

Matthew Walker:  Very much so. Tough love all the way.

Chris Kresser:  Yes, yes. Do you think you might have done an MD/PhD program if those existed at that point, 20/20 hindsight?

Matthew Walker:  Very much so. Yeah. I think what I love about science is that it’s intellectually incredibly fulfilling. But emotionally, an academic life, in some ways is perhaps a little bit devoid of emotional fulfillment. Because it’s almost egotistical. What grants are you getting? What podcasts are you on?

Chris Kresser:  Right. Where are you publishing?

Matthew Walker:  What books, yeah, where are you publishing? What awards have you won? And that didn’t really, for me, at least personally, it didn’t really satisfy. And where I get my emotional nourishment from is actually teaching. I adore teaching. I teach a big class here at UC Berkeley on the science of sleep. And there, it’s not about me, it’s about my students. And that is very ego-less, and you can’t have an ego in that regard. Not if you want to be a good teacher.

Chris Kresser:  Right, right. We’re neighbors, by the way. I’m up in North Berkeley Hills. And I did my undergrad at UC Berkeley.

Matthew Walker:  Really? Okay. And your undergrad was in?

Chris Kresser:  Berkeley being Berkeley, I made up my own major. So it was, I wrote my thesis on the social and cultural implications of the internet, which was just emerging. You and I are about the same age. So this was ’92 to ’96.

Matthew Walker:  My goodness, you’re prophetic.

Chris Kresser:  Well the World Wide Web was like, gray background pages with blue links, you remember that? And my claim to fame was that I had a picture on my website, which was pretty, pretty fancy.

Matthew Walker:  Yeah, I remember sort of doing internet sort of communication, which was the forerunner of email and working in terminals and scripting.

Chris Kresser:  Right, right. Exactly. So you teach a big course at Berkeley.

Matthew Walker:  I do.

Chris Kresser:  Wow, that’s phenomenal.

Matthew Walker:  Yeah, it’s called The Science of Sleep. It’s about 500 or 600 kids.

Chris Kresser:  Oh, I love it. That’s great, I remember those courses. I’m sure they love it.

Matthew Walker:  Such a privilege.

What We’ve Learned about Sleep

Chris Kresser:  Let’s talk a little bit, you mentioned, I certainly remember the time when the question “Why do we sleep?” was an enigma. And no one really understood the answer. So what have we learned in that previous 20-year period about how humans are wired to sleep biologically? Why do we sleep in the first place? And then I’m also curious to know, has the answer to that question changed as we progressed from our hunter–gatherer roots through to the development of agriculture, and then more recently, industrialization, and even more recently, the advent of screens and other technologies that have disrupted our circadian rhythm? Certainly the most, most recently, that’s too fast for genetic evolution to occur, but perhaps not epigenetic changes. So I’m curious about that whole trajectory.

Matthew Walker:  Yeah, so, great questions. Let me sort of try to reverse the reel. I think the first question is, why do we sleep? And we didn’t have a good answer 20 years ago. The crass answer was that we sleep to cure sleepiness, which is the fatuous equivalent of saying that we eat to cure hunger.

Chris Kresser:  Ontological argument.

Matthew Walker:  Right, yeah. It tells me nothing about the nutritional benefits of nutrition and of food and so. But now, after a remarkable litany of research of the past sort of quarter century, we’ve actually been forced to upend the question and ask, “Is there anything that sleep does not provide in terms of a functional benefit both for brain and body?” And it’s been a real struggle. We found that most all tissues and physiological systems in the body and most all operations of the mind are powerfully enhanced when we get sufficient sleep and quite demonstrably impaired when we don’t get enough. So I think that that’s been a radical change of events.

I think moving on to your question about sleep as a function of evolution and for human beings, I think the question about sleep evolving is interesting for at least two reasons. Firstly, if you take a step back, it is the most idiotic of all things to do as a biological species. Because, firstly—

Chris Kresser:  It’s kind of dangerous, right?

Matthew Walker:  Yeah, you’re not finding food, you’re not reproducing, you’re not finding a mate, you’re not caring for your young, and worst of all, you’re vulnerable to predation.

Chris Kresser:  Yeah.

Matthew Walker:  So, on any one of those grounds, and especially all of them as a collective, Mother Nature should have strongly selected against this thing called sleep. So in other words, if sleep doesn’t support an absolutely vital set of functions, then it’s going to be the biggest mistake the evolutionary process ever made. Now we understand it wasn’t a blunder. It was a sublime stroke of genius. But the second reason I think that question is interesting is perhaps because sleep didn’t evolve. We assume that it did.

And I’ve got a wacky theory that I think is going to be very difficult to prove. And I’m sure I’m probably wrong. But I have an idea that, in fact, we started asleep. And it was from sleep that wakefulness emerged. Why do we assume that wake was the predecessor and then we have to evolve this thing called sleep? I think we started off sleeping, and from that, wakefulness emerged. And wakefulness is, in many ways, remarkably deleterious. If you look at the brain, it’s pretty clear metabolically, wakefulness is low-level brain damage. And so I think we had to return back to the sleep state every time we emerged awake.

So, in some ways, I almost don’t think that sleep did evolve in terms of an inception point. But then sleep has evolved dramatically across species. Every species that we’ve studied appears to sleep. Even very old, evolutionary, ancient earthworms. Bacteria, in fact, even have an active and a passive phase, which seems to be maybe a precursor of sleep and wake. And from that point forward, sleep has fought its way through, heroically every step along the evolutionary path. How could it, if it’s that well preserved, how could it not be essential?

Chris Kresser:  Yeah, makes a lot of sense to me. And I love your inquiry into that and how you’ve turned it around. Because sometimes that’s where the greatest insights come.

Matthew Walker:  Or the most idiotic of them. We’ll see.

Chris Kresser:  Yeah, we’ll see. So let’s talk a little bit more about what happens when we’re sleeping, specifically from a physiological perspective.

How much sleep do you need? Can a nap make up for lost sleep? Is there really a difference between night owls and morning larks? Check out this episode of RHR for answers to these questions, and learn five things you can do to improve your shuteye. #healthylifestyle #chriskresser

How Our Sleep Patterns Have Changed

Matthew Walker:  Yeah, and, oh, actually, I realized I didn’t answer the second part of your question, which was about how sleep has changed.

Chris Kresser:  Has it changed, yeah.

Matthew Walker:  Yeah. Gosh, it’s a great question. So, yeah, it has seemingly changed as we’ve moved from sort of a hunter–gatherer way through to the agrarian sort of revolution, then the Industrial Revolution, and now we’re in the digital revolution. If you go back and you study some of these tribes that we’ve done, these hunter–gatherer tribes whose way of life hasn’t changed for thousands of years, I think the two big take-home messages for sleep are the following. Firstly, they don’t sleep the way that we do in terms of timing. In other words, have you ever thought about what the term midnight actually means?

Chris Kresser:  Middle of the night.

Matthew Walker:  Middle of the night, right, which for us, midnight in this modern era is usually the last time that we check email.

Chris Kresser:  Bedtime.

Matthew Walker:  Yeah, bedtime. But in fact, if you look at these hunter–gatherer tribes, that really is the middle of their night. They typically go to bed a couple of hours after sundown, that’s their midpoint of sleep. And then they typically start to rise a little bit before dawn. It’s usually that they rise with the inflection of temperature as it increases rather than the actual sun. So it’s temperature that seems to help us better wake up necessarily than light itself, although light does have that function.

So that’s the first thing, is sort of where they place their sleep on the clock face is different to the way that we do in modernity. The second way is the phases of sleep. They typically have what’s called biphasic sleep. In first-world nations, we do essentially monophasic sleep. We sleep in one bout throughout the night. We try to get our eight to nine hours, hopefully, in that way. They don’t do that. They typically sleep somewhere between sort of six and a half to seven hours at night. And then they, in the afternoon, they typically take a nap, a siesta-like behavior to make up the remaining, on average, eight-hour necessity that all human beings seem to need. So I think that that’s one of the sort of realizations.

Now, moving through into sort of the timeline of human evolution, we did see a strange behavior. It was more around the sort of the Dickensian era where people would have this thing called first sleep and second sleep, where they would sleep for about four hours, then they would wake up in the middle of the night, they would socialize, make music, eat, drink, make love. And then they would go back down and have what’s called a second sleep. Did that happen? It seems to from the writings, historically. Is that physiologically the way that we were designed to sleep? No, there’s no good biology that tells us that. It was probably a social propagation of a sleep behavior, not a biological edict.

Chris Kresser:  I have some patients who’ve read some things online about this. And I asked them about their sleep patterns. They tell me they’re doing biphasic sleep. And I do some digging and find out that they’re waking up at three in the morning, two in the morning and sending emails and surfing the web. And then I asked them, “Okay, well, are you also going to bed at eight o’clock at night, which is part of that whole equation?” No, not doing that. So yeah, it seems, even if it was typical at a certain period of time, that doesn’t mean it’s normal for humans, or beneficial for human physiology.

Matthew Walker:  Yeah. And I think that’s a very understandable thing to consider for the public, when they hear this, that it was a common practice thing. Well, okay, that’s how we should, that’s how we were designed to sleep. But of course, the timeline of Homo sapiens, it goes back much further than that. And if you look, that’s not how we were designed to sleep.

Chris Kresser:  Yeah. So I want to come back to what happens physiologically with sleep. But you mentioned something that I’m sure perked, a few people’s ears perked up at, which is that all humans, or most humans, that’s actually part of the question, need eight hours. And you mentioned eight to nine hours as well. So that’s obviously one of the most commonly asked questions is how much do I need to sleep? And so that’s part one, and part two is how do you know if there is a range for humans? How do you know where you are in that range?

Matthew Walker:  Yeah, two great questions. So firstly, what we know is that the number of people who can survive on seven hours of sleep or less without showing any biological or cognitive impairment, rounded to a whole number, and expressed a percent of the population is actually zero.

Chris Kresser:  Wow, wow.

Matthew Walker:  And I think some people get caught off guard because they’ve heard of this sort of short, this selection of individuals that have this sort of short-sleeping gene. And we know this gene, it’s called the DEC2 gene. And when I sort of describe this short-sleeping gene, lots of people think, “Oh, I think I may be one of them.”

Chris Kresser:  Of course.

Matthew Walker:  It’s a fraction of the population. You’re much more likely statistically to be struck by lightning then have this gene, just an FYI. And by the way, they don’t sleep five hours. If you look at the data, if you bring them into the laboratory, and you say, “Just let them sleep,” you take away all clock faces, complete darkness in the lab, they have no cue. It’s just, “What is their habitual natural sleep expression?” On average, they settle into around about six and a quarter hours. So a genetic short-sleeping mutant sleeps six and a quarter hours. That’s the shortest sleep variable. And of course, for most people, we know from surveys that the average American adult is sleeping around about six hours and 31 minutes.

Chris Kresser:  Right.

Matthew Walker:  And they aren’t a genetic mutant.

Chris Kresser:  No. And a third of Americans are getting fewer than six hours of sleep.

Matthew Walker:  Correct. So that’s the average. Yeah, and your point is an elegant one. If you think about that distribution, a large proportion, at the tail end of that, definitely much less than that.

Chris Kresser:  It’s kind of like everyone thinks they’re a phenomenal driver, or when the surveys come up, it’s everyone’s an awesome driver. They’re an incredible love maker.

Matthew Walker:  That’s right. I believe that was Stirling, I’m obsessed as a racecar driver myself. And so I think the thing that was the, the thing that Stirling Moss said, the famous British racecar driver, “All men think that they are great drivers and wonderful partners too.”

Chris Kresser:  That’s right. That’s right. And we all think we’re short sleepers, right?

Matthew Walker:  Exactly.

Chris Kresser:  We’re the sleep mutants.

Matthew Walker:  Yeah, au contraire.

Chris Kresser:  But that you’re more likely struck by lightning than to be someone with this gene. And even then, it’s required, you still require a level of sleep that 35 percent of Americans aren’t getting.

Matthew Walker:  That’s right. And we also, by the way, don’t know if that’s the sleep that they get. We don’t necessarily know if that means that they have a shorter lifespan. Because we’ve only just discovered this genetic alteration. We haven’t had the chance to track these individuals, we don’t really know. There’s too few of them to get sort of good data, to say from a metabolic perspective, is their glucose regulation worse or cardiovascularly, do they have hypertension more frequently or do they have a high cancer risk? We actually just don’t know. So yes, they may sleep less, but does that mean that they survive just as long in both lifespan and healthspan? We have no idea.

Chris Kresser:  Right.

Matthew Walker:  So that’s another assumption.

Chris Kresser:  I’ve also read some research, I’m sure you can enlighten us on it, that there’s a difference between subjective and objective impairments in sleep. So that when people are sleep deprived, they initially will report subjective changes in their, in various competencies. In other words, they notice that they’re sleep deprived and it’s affecting their performance, their cognition, etc.

But after some period of adaptation, they stop being aware that their performance is impaired, even though with objective measures, it can be clearly demonstrated that it is. And so this leads to another problem where people think, “Hey, I’m fine. I’m getting six or five hours of sleep, but it’s not affecting me at all.”

Matthew Walker:  It is one of the big dangers with sleep is that you don’t know you’re sleep deprived when you’re sleep deprived. So your subjective sense of how well you’re doing is a miserable predictor of objectively how well you’re doing. So the analogy there would be a drunk driver at a bar. They’ve had seven or eight shots and a couple of beers.

Chris Kresser:  “I’m fine.”

Matthew Walker:  They pick up their keys and they say, “I’m fine to drive home.” And your response is, “No, subjectively, I know you think you’re fine to drive home, but objectively, you’re not.” And the same is true with a lack of sleep. And it’s one of the real, I think, sort of concerning components of that. A great example of this too, a clinical example, is in a sleep disorder called sleep apnea, where you have severe snoring. And that’s a very, it’s a very gradual descent into sleep deficiency.

So it happens almost over years that you develop this real chronic sleep restriction. And people with that disorder, most of them are undiagnosed and untreated. They just think, well, this is me at this age. This is just how I am, this is how my performance is. But when you treat them, their health metrics are remarkably changed for the better. And they just don’t realize how well they could feel once the sleep was restored. And I think it’s a great clinical example of this. A patient once said to me it was almost as though someone wiped a fogged glass clear and finally I could see the world, comparatively. And it’s that radical a transformation. I think that’s what a lot of people are experiencing right now. They’re carrying a chronic sleep debt and they truly don’t know how good a version of themselves mentally and physically, they could be if only they slept enough.

Which I think comes onto your second question, which was “How do we know how much sleep that we need?” I think the first and obvious question, or answer, sorry, to the question would be, if you didn’t set an alarm, would you sleep past your alarm? And if the answer is yes, then you’re not getting sufficient sleep. Your body and brain are not done yet with sleep, and you clearly need more. Now, that’s one good metric. It doesn’t mean, however, that for some people who suffer sleep maintenance insomnia, where they’re waking up throughout the night, or they’re having unfortunate early morning awakenings, that doesn’t mean that they are well slept. So it’s not a complete sort of pure diagnostic test. But for many people, it’s a very good metric. What is the range of sleep? We see a range in the general population of seven to nine hours of total sleep time is needed. And I emphasize “time” because time is not time in bed. I often say that you need an eight- to nine-and-a-half-hour sleep opportunity to get seven to nine hours of sleep. Because most people when they turn off the light, they don’t fall asleep immediately and then stay asleep for the entirety of the night. It’s called your sleep efficiency.

So in other words, of the total amount of time that you’re in bed, what percent of that time was asleep, and that’s your percent sleep efficiency score. And if you’re healthy, you usually have a sleep efficiency of above 85 percent, hopefully a little bit better than that. Now, if you think about an eight-hour night of sleep, which if I do the math, I’ll do it wrong, but it may be 440 minutes, and you think you’re at a 90 percent sleep efficiency, that means that you’re going to be awake for 44 minutes throughout that night. So for you to get your eight hours, you actually have to be in bed for eight hours and 44 minutes.

And people make this mistake where they hear scientists like me say, “Oh, you need seven to nine hours of sleep,” they think that’s seven to nine hours in bed. So if I get into bed for seven hours, you’re actually probably much more likely to be getting somewhere around six and a half hours of sleep, if not less. So that’s, I think it’s a really important point for people that they don’t typically realize, because scientists like me don’t really explain that very well.

Chris Kresser:  Such important points, and I think a lot of people are probably freaking out right now listening to this.

Matthew Walker:  You’re welcome.

Chris Kresser:  But it’s good. I mean, it’s, I think, having the right information is more than half the battle in terms of knowing. Well, it’s half the battle, let’s say. Changing behavior is a whole other ball of wax.

Matthew Walker:  Yeah, and a harder one maybe too.

Tracking Your Sleep

Chris Kresser:  The information is definitely not enough to change behavior. But I think a lot of people simply are not aware of what you just said. So it’s partly a question of lack of awareness of how much sleep is actually needed for optimal health and performance. And then it’s also a question of how the heck am I going to find nine and a half hours to be in bed when I’ve got a family and a job and everything else related to the modern lifestyle.

But since we’re talking about tracking sleep, things like sleep efficiency ratio and how long you’re actually sleeping when you’re in bed. A lot of people aren’t, that’s not going to be obvious to them, because they’re sleeping, or they’re tossing and turning and maybe not sleeping. So what are your preferred methods of tracking sleep for consumers who aren’t able to be in a lab at this point in time?

Matthew Walker:  Yeah, it’s difficult. I think most of the devices out there right now, sort of the wrist watches, or the rings that you can wear, they do a pretty good job of differentiating basic total sleep time. The problem is that they’re not very good at separating out all of the different unique sleep stages that I could do at my sleep center here at Berkeley. So I think right now they’re probably hovering around 65 to 75 percent accuracy. And I think we will get better …

Chris Kresser:  Compared against polysomnography?

Matthew Walker:  That’s right. So, polysomnography is essentially doing a sleep study. You look like a spaghetti monster with all of these wires coming off your head and your face, but it allows us to measure your sleep at gold standard accuracy. So that’s relative to that gold standard. Will we get there? I think we will. In terms of, so, I think one issue is accuracy that we’ve yet to solve for.

The second is just form factor. I’ve become very enamored with a concept I would describe as “unwearables” when it comes to sleep tracking. Because when we go to bed, we typically take things off, we don’t put things on. And that’s why I don’t think these headband devices are probably going to be very sticky. For most people, I think you have to be very sleep committed and curious to sort of maintain that long term. I think wristwatches probably feel a little bit foreign to some people, and that’s why adherence for sleep usually decreases. I think the ring is a better form factor for that because people do go to sleep with wedding rings and other rings on and so I think it’s a less intrusive component.

And then finally, I think probably the best hope that we have out there right now is the mattress. I think that has the capacity to be probably one of the better sleep trackers. And it’s surprising to me that we haven’t done more in this space. We, if you think about the car that you have right now relative to a car of the 1950s your car now is packed full of incredible sensors that are always measuring the health and the status of that car. If you think about your mattress in 1950 relative to your mattress now, they’re much more similar than they are different.

Chris Kresser:  Right.

Matthew Walker:  Why is that? And you think about the car, if you’re unlucky, you may spend, I don’t know, 5 to 10 percent of your life in your car. Well, you should be spending 30 percent of your life in your mattress. But yet, we don’t. “On” your mattress, not “in.” If you’re in the mattress, there’s a different problem. But I think there’s this disproportionate technology bias and also spending bias towards these things. So I hope that sort of breaks down the tracking regime. I think we’re going to get there with accuracy, and then sort of fans out the form factor that I think is best as well.

Chris Kresser:  I mean, that’s a great point, we’ve got, like, Bluetooth and Wi-Fi in coffee makers, for crying out loud. You can use your phone to start making an espresso. But the mattress is relatively unchanged, and it’s such an important piece of our lives.

Sleep Patterns of Night Owls and Morning Larks

Chris Kresser:  So we’ve talked a little bit about optimal duration of sleep and what amount of sleep most people need. Does it matter when sleep occurs? So you hear people say, I’m a night owl. So, I go to bed at two and I wake up, even if they’re getting eight hours of sleep, let’s say they wake up at 10. Is that any different physiologically, than going to bed at nine and waking up at five?

Matthew Walker:  It’s a great question and it’s a really under-discussed question, I think. So, as you mentioned, some people are night owls, others are morning larks, and other people are somewhere in between. And this is what we call your chronotype. So, are you a morning type, an evening type? And about a third of the population is a morning type, about a third is an evening type, and the other third is sort of somewhere in between. Why is this important?

Well, it is important because it does seem to make a difference to the quantity and quality of your sleep. So, an example would be I often get sort of both patients and people sort of speaking to me and saying, “Look, I am, I’m getting into bed and I have terrible insomnia. I can’t fall asleep for the first hour and a half.” And then I’ll say, “Okay, what time you going to sleep?” And they would say, “Well, I go to sleep at 10 p.m. because I have to wake up at 6 a.m. to get to work.” And then I’ll say, “Well, what if you were on a desert island and you had no commitments, nobody thought to wake up for nothing? When would you just like to go to bed? And when would you like to wake up?” And they would say, “Well, I’m actually more of an evening person, I’d like to probably go to bed around 1 a.m. and maybe sort of wake up at nine or 10 a.m. the following morning.”

What’s happening there is that they probably don’t have insomnia. They have a mismatch between the enforced sleep window, and their natural biological chronotype. In other words, they’re getting into bed when their biology and physiology is not yet ready to ramp up this thing called sleep. And it masquerades as sleep-onset insomnia, difficulty falling asleep.

So I think it’s important to sort of try to ask yourself, what type are you? And then ask, “Is there a chance that I could try and alter my schedule to fit better with that?” Now, that’s very difficult, I understand, for many people with lots of responsibilities, such as jobs and family. But even if you can’t, I hope that information at least helps you perhaps understand why some of your sleep may not be as ideal as it could be.

Chris Kresser:  Right. And is, I’ve read that this is also an issue with teenagers, this chronotype change throughout life. And if a teenager naturally, the chronotype shifts where they are going to sleep later and waking up later. There’s been a movement in California, I think Gavin Newsom is supportive of it, to make school start times later for kids on the basis of some of this research.

Matthew Walker:  Yes, and I think it is a critical movement to push forward. Firstly, we know that your chronotype is genetically defined. So at the point of birth, in fact, of course, before, you don’t get a choice as to whether you’re an evening type or a morning type. It’s gifted to you by your genetics. And as with most biology, when you fight biology or genetics, you typically lose. And the way you know you’ve lost is through disease and sickness. And so I think that’s the first thing to realize. It’s not under your control, really.

So don’t blame yourself or beat yourself up for being an evening type. Because society, really, I think, quite stigmatizes evening types as being lazy. The early bird catches the worm or this sort of nonsense. So yes, we’re all gifted with our chronotype. But our preference for sleeping phase does change across the lifespan. When we’re young kids, even though we’d love to stay up, even if we’re an evening type genetically defined, you still will struggle because you’ll still overall when you’re young be getting to sleep earlier and waking up earlier, much to the chagrin of young parents on the latter. But then as we transition through adolescence, there is a remarkable shift in your spots called your circadian rhythm, your 24-hour rhythm, to where you want to go to bed later and wake up later.

And again, it’s not the teenager’s fault that they, they, this, it’s just part of their preprogrammed biological development. And then as we get into adulthood, we kind of find our sort of overall adult sweet spot. But then oddly, as we go into older age, that circadian rhythm starts to get dragged backwards, and we start to want to go to bed earlier and then wake up earlier. And that’s the reason in Florida, you’ve got things like the early-bird specials, and that is because of this biological change. So, for adolescence though, it is critical. The data is very clear on this.

When kids start going to school later, kids of all ages, but especially teenagers, firstly, we see that academic grades increase. But we also see that behavioral problems decrease, truancy rates decrease, psychological and psychiatric referrals also decrease. But then something else was strange that happened in that story of later school start times around the country, which was that the life expectancy of those students increased. And you may be thinking, well, how can you, how do you measure that? How do you understand that? The leading cause of death in late-stage teenagers in most developed nations is actually not suicide. That’s second. It’s road traffic accidents.

Chris Kresser:  Right.

Matthew Walker:  And here, sleep matters enormously. And I’ll just give you one example that happened in Teton County in Wyoming. They shifted their school start times from a rather ridiculous 7:35 in the morning, to 8:55 in the morning. Now, in addition to those kids reporting getting an extra hour of sleep that following year, what they ended up seeing was a 70 percent drop in car crashes in those teenagers that following year.

To put that in context, the advent of ABS technology in cars, which is antilock brake systems, that dropped accident rates by 20 to 25 percent, and it was deemed a revolution. But here is a biological factor, giving our next generation sufficient sleep, that will drop accident rates by 70 percent. So I think, I really hope Gavin passes the bill here in California, just like has happened in other states. Because if our goal as educators truly is to educate and not risk lives in the process, then we are failing our children in the most spectacular manner with this incessant model of early school start times.

Chris Kresser:  Absolutely. And as parents and protectors of the health of future generations, this is a serious moral and ethical issue.

Matthew Walker:  It really is.

Chris Kresser:  Yeah.

Matthew Walker:  It really is. When sleep is abundant, minds flourish. And when it’s not, they don’t.

How Naps, Meditation, and Medication Affect Sleep

Chris Kresser:  Yeah, it’s as simple as that. Okay, so we’ve talked a little about chronotypes, different durations, timings of sleep. The next question is napping and catching up on lost sleep, if, for example, you’re someone that can’t for whatever reason, get the appropriate amount of sleep during the night. How do you make that up? And I mean, does that work, for example? Does napping replace sleep debt through the night? Maybe let’s start there. And I have a few other follow-up questions.

Matthew Walker:  Yeah, so, it’s a little bit complicated. But let’s pick it apart. Firstly, what we know is that sleep is not like the bank, retrospectively. You can’t accumulate a debt and then hope to pay it off at some later point in time. So, for example, if you under-sleep one night, your immune system is going to be compromised the next day. And for that day, whilst you are compromised, you’re going to be suffering higher risk of infection, as well as things like cancer.

But when you get some recovery sleep that following night, it doesn’t remove or go back in time and sort of wipe away that vulnerability. You’ve suffered that vulnerability, there’s nothing you can do. But it doesn’t mean that you can’t de-risk the future by then sleeping enough and getting that sleep. So, but, in terms of naps, the reason why it’s a little bit tricky is, as I mentioned, for hunter–gatherer tribes, they make up on a 24-hour basis their sleep in a different way than we do. They sort of sleep a good solid bout at night and then they have a nap/siesta-like behavior in the mid to late afternoon.

So they get their fill of sleep, but they do it consistently. They’re not short sleeping during the week and then bingeing at the weekend, which is what typically we see, which is almost what I would describe as sort of the “sleep bulimia,” this sort of binge–purge sort of approach to sleep. And that’s not good. It creates what we call social jet lag, which is where on Friday and Saturday, maybe you’re going to bed at midnight or one and you’re waking up at 9, 10, 11.

But then come Sunday night, you’ve got to drag your biological clock all the way back to going to bed at 10, and you’re doing that every weekend, that’s essentially like flying back and forth from New York to San Francisco every week.

Chris Kresser:  Right, it’s self-imposed shift work.

Matthew Walker:  And that’s torture on your body. Yeah, it is, it is a form of social shift work, social jet lag. And we can see, there’s been some great studies showing the health consequences of that behavior. So that’s really the first piece of advice I can give you for better sleep is regularity. Go to bed at the same time, wake up at the same time, no matter whether it’s the weekday or the weekend. Regularity is king. It’s what our bodies were designed to receive. And it will help anchor your sleep and improve both the quantity and the quality of that sleep.

Chris Kresser:  So it sounds like napping, assuming you’re asleep, does count towards the total amount of sleep you need in a 24-hour period. What about meditation?

Matthew Walker:  Well, I should note, by the way, for naps. Naps can be a double-edged sword. When we’re awake during the day, we build up a healthy sleepiness, and it’s due to a chemical called adenosine. And the more of that adenosine that builds up, in other words, the longer that you’re awake, the sleepier you will feel. And after about 16 hours of being awake, you should feel plenty sleepy. You should have enough of this sleepiness pressure or adenosine chemical to weigh you down so that you fall asleep easily and then stay asleep. And it’s during sleep that the brain actually takes away that adenosine chemical. It removes it and therefore we wake up after about eight hours feeling refreshed, or we should do.

So why does this make any difference to naps? Well, if you’re taking a nap in the late afternoon, it’s almost like a pressure valve cooker. And a nap will just release some of that healthy sleepiness. And therefore, you may struggle to fall asleep at your normal time at night, and/or stay asleep. So the advice is, if you can nap regularly, or you don’t have problems with sleep at night, then naps can be okay. But if you can’t nap regularly, and especially if you struggle with sleep at night, you should not nap. You should stay awake, build up all of that healthy sleepiness to help you sleep better at night and stay asleep, if that makes sense.

Chris Kresser:  It does, yeah. And I think that’s an important point, especially for people who don’t sleep well. Because they are the very ones who might think that napping could benefit them.

Matthew Walker:  Right. Yeah, you can fall in, and it’s a very easy trap to fall into. But you should really resist that. It’s something that we strongly advocate.

Meditation, great question. Meditation seems to give some similar benefits to sleep for both brain and the body. But what’s interesting is that if you put people into a brain scanner, or you measure their actual brainwave activity, as we’ve done, meditation does not produce a brain state that is similar to sleep.

So I think my understanding right now for meditation versus sleep, if you want to sort of put them on a Coke/Pepsi head-to-head challenge, yes, they both give benefits. Perhaps similar benefits. But the physiological route through which they accomplish and achieve those benefits seems to be different. And as a consequence, meditation is not a substitute for sleep. Is meditation, however, beneficial to help you sleep? It actually is. So for people who are struggling with sleep at night, meditation has actually been proven to be very efficacious. The studies are really quite powerful.

So it’s one of the tools that people are increasingly using clinically to try and help people sleep. And if you look at some of these popular meditation apps, like Calm or Headspace, and you sort of try to inquire about the usage statistics, they would obviously start it off by thinking we would like people to meditate in the morning or during the day. Where they saw a lot of uptick in the use of these apps was right around bedtime, and people were self-medicating their sleep troubles with meditation.

Chris Kresser:  Yeah, self-meditation.

Matthew Walker:  Yeah, self-meditation/self-medication. So I think meditation, that’s really the relationship in terms of sleep that’s become interesting.

Chris Kresser:  Interesting.

Matthew Walker:  Less so, if you meditate, can you get less sleep? The answer seems to be no, because they’re two physiologically different processes.

Chris Kresser:  Right, that’s helpful. So meditation is a lever, one of the things you can do to improve your sleep, but not something that can replace sleep.

Matthew Walker:  That’s right.

What You Can Do to Improve Your Sleep

Chris Kresser:  So, I think a lot of people are aware of the importance of sleep, at least in my audience. They’re aware of the importance of sleep, they, whether or not they’re struggling with finding enough time to be in bed, I think many people are. But then there’s the issue of insomnia. People who are actually spending the required amount of time in bed, but they are suffering from insomnia for any number of reasons. And we don’t have time, obviously, to go into all of the causes of insomnia, and they can vary a lot from person to person.

But in terms of just general sleep hygiene and general tips for developing better sleep habits, what are the things that you have seen from the research and also your work and the Center for Human Sleep Science that make the biggest impact for the greatest number of people?

Matthew Walker:  Yeah, so, I think for insomnia, just to put a sticky on it, there are essentially two flavors that we think about for insomnia, which is sleep-onset insomnia versus sleep-maintenance insomnia. In other words, I have difficulty falling asleep versus I have difficulty staying asleep. Right now, the treatment recommendation is not first-line treatment with sleeping pills. Unfortunately, the evidence is growing. Those sleeping pills seem to increase your risk of mortality significantly. They’ve also been associated with a high risk of cancer development.

Do we know that those studies are causal? No, we don’t. They could be associational. Although there is some animal work to suggest it could be causal. Will we ever find out if it’s causal? Based on the strength of the relationship between mortality and the carcinogenic nature of those, I think it’s probably unlikely that we would ever get ethical approval to do studies. That’s how strong that evidence is.

Chris Kresser:  Right.

Matthew Walker:  So sleeping pills in 2016, the American College of Physicians said on the basis of that, plus the nominal benefit that you get relative to placebo, the first-line treatment must not be sleeping pills. It must be something called cognitive behavioral therapy for insomnia, or CBTI. And there you work with a clinically trained therapist. The benefits on sleep are just as big, if not bigger, than what typically you see in terms of sleeping pills.

But more beneficial is that once you start working with that therapist, you maintain that benefit, some studies have shown up to a year, even five years, most recently. But yet, if you stop taking sleeping pills, not only do you go back to the bad sleep that you were having, you actually have what’s called rebound insomnia, where your sleep is even worse. So I don’t think sleeping pills are really the answer, at least not the ones that we have right now.

Chris Kresser:  And this is true across all of the classes of sleep medications.

Matthew Walker:  Unfortunately.

Chris Kresser:  … Lunesta, Ambien, Sonata, etc.

Matthew Walker:  Yeah, they all do essentially the same thing. They’re all trying to essentially tickle a type of receptor in the brain that’s called the GABA receptor. And that’s the principal inhibitory receptor of the brain. What those drugs essentially do is knock out your cortex. And so they’re a class of drugs that we call the sedative hypnotic. And sedation is not sleep. But when you take these drugs, you mistake the former for the latter. Now, I’m not saying that when you take those sleeping pills that you’re awake. You’re clearly not. But to argue that you’re in naturalistic sleep is an equal falsehood.

Chris Kresser:  Right.

Matthew Walker:  And if I were to show you your electrical signature of sleep, when you’re on versus off those drugs, it looks very different.

Chris Kresser:  Do they increase light sleep and decrease deep sleep, these drugs?

Matthew Walker:  They can, it depends on how you quantify that sleep. If you just score the sleep itself, it looks more like a sedated sort of deep sleep. But if you look at the electrical profile, which is really the best metric that we have of the quality of sleep, it doesn’t look like good-quality sleep. So, and I think also people have issues with tolerance, they typically can experience a sort of a hangover effect in the morning as well, even with some of the more shorter-acting compounds.

I think the bottom line is that sleep, if you understand the biology, is an incredibly complex, physiological ballet of different things. And to think that we could orchestrate that sort of ballet with just a simple basic drug is probably unlikely. That’s what we’re finding. Our sleep solutions pharmacologically are going to have to be pluripotent. We’re going to have to have multiple different skews by which they try to sort of improve sleep. They have to pull all of the different levers at all of the right times in all of the right brain areas to really mimic naturalistic sleep. So I think we’re a little bit off that in terms of solutions, but we’ve got a great behavioral alternative, which people can turn to.

Chris Kresser:  Right. And yet 50 to 70 million Americans are taking these medications. And they unfortunately, all too often are prescribed as a first-line therapy. And to me, and what strikes me about this, like we talk on this show and in general a lot about the ancestral or evolutionary perspective. And it seems to me that several aspects of the modern lifestyle are just antithetical to good sleep.

From circadian disruption with too much exposure to artificial blue light at night that impacts melatonin production, and not enough exposure to sunlight during the day, to the kind of always-on lifestyle that many of us live where we’re checking email, social media all times during the day. I know you’re well aware of all these statistics of what percentage of teens, especially, but even young adults, and adults in general, check their phone throughout the night and sleep with it in their bedroom. To the higher incidence now of chronic inflammatory diseases, because I know from my work with patients, that inflammatory disorders are very likely to interfere with sleep on like just a physiological basis, even if someone’s mind isn’t busy.

So what’s your impression of that and the challenges that the modern lifestyle presents in terms of sleep? How much of this is just evolutionary mismatch?

Five Things You Can Do to Improve Your Sleep

Matthew Walker:  Yeah, my impression, I actually don’t do impressions. My training is strictly in science. But I think to come back, you sort of asked me, what would be my tips for those who don’t have clinical insomnia to try? And I think it speaks to this issue to what would be, let’s say, the five things that people could start doing tonight to get better sleep.

1. Set a Regular Sleep Schedule

The first, we’ve already spoken about, which is regularity. That really is, if there’s only one thing that you remember from this sort of podcast for sleep, it would be regularity, and especially the wake-up time. Really try to anchor that, and the rest of the regularity will almost take care of itself.

2. Control Your Temperature

The second component of that is temperature. So we know that with sort of this modernity shift, indoor living, we’ve also generated constant temperatures in our homes with air conditioning and central heating. That has been a remarkably difficult thing for our sleep system because your body needs to drop its core temperature by about two, two and a half degrees Fahrenheit, or about one degree Celsius, to fall asleep and then stay asleep. And it’s the reason you will always find it easier to fall asleep in a room that’s too cold than too hot. Because the cold is taking you in the right direction.

So a temperature around about 67-ish Fahrenheit is about optimal for most people. Now, you can play around with it, but it’s a lot colder than most people think. You can, if you get cold hands and feet, actually prevent yourself from falling asleep, because the way it works is that your hands and your feet radiate heat from your body. And you actually need to expel the temperature from the core of your body. How do you do that? Well, your hands and your feet are wonderful peripheral radiators of heat.

And this is the reason why when you get into a hot bath or a shower, you think that you sleep better. It’s actually not because you’re coming out nice and toasty. It’s because in the bath, you get massive vasodilation. All of the blood comes to the surface, you get rosy cheeks. That when you get out of the bath means that you have this enormous thermal dissipation of heat from the core of your body, which drops your core body temperature, and that’s why you fall asleep easier. So you can have a hot water bottle at the end of your bed, you can go to sort of sleep with some socks on to begin with to warm the feet to then cool the core of the body.

It’s paradoxical, but that’s how it works. But then, and then you need a nice, cool ambient temperature. Because you not only need to sort of essentially cool down to fall asleep, you need to stay cool to stay asleep. And that’s where the ambient core temperature is going to help you.

So the second, sorry—

Chris Kresser:  That’s temperature. The second?

Matthew Walker:  The second is temperature.

Chris Kresser:  That has made the biggest impact on my sleep. I have been using the chiliPAD, or the OOLER, which you may be familiar with, the cooling sleep surface.

Matthew Walker:  Yeah.

Chris Kresser:  And we have one of the few houses I think in Berkeley I think with air conditioning, which has been, for me, when I go to the tropics or something, my sleep is just destroyed from being in that hot environment. So temperature is number two, regularity is number one.

Matthew Walker:  Temperature, number two, yeah.

Chris Kresser:  And how about number three?

3. Keep It Dark

Matthew Walker:  Number three is darkness. We are a dark-deprived society in this modern era. So trying to shut down those screens in the last hour before bed. We know that that has a deleterious impact. There was a study that was done where you did one hour of iPad reading before bed versus just reading a normal paper book in dim light. And that one hour of iPad reading, firstly, it reduced the amount of this sleep sort of timing chemical called melatonin. It dropped that melatonin level by 50 percent.

Chris Kresser:  Wow.

Matthew Walker:  Furthermore, it delayed the peak of that melatonin release by three hours. So if we did an hour of iPad reading here in Berkeley before bed, we would be much closer to Hawaii time than we would be California time.

Chris Kresser:  Right.

Matthew Walker:  And what was also interesting is that when they stopped, it also blocked their rapid eye movement sleep, by the way, their dream sleep, which we know is essential for a number of functions. But the more fascinating thing was, when they stopped the iPad reading, they didn’t immediately return to good sleep. It actually had a blast radius, where the sleep didn’t recover until two or three nights later. So I think darkness is a critical component.

Again, it’s a way that we’ve been dislocated from our natural sort of physiological interaction with the environment when it comes to sleep through the Industrial Revolution and now the digital one. It’s not just devices though, it’s also just overhead lighting. So in the last hour before bed, try to dim down half of the lights in your house. You don’t need them all on, and you’d be surprised at how soporific that actually is. So that’s number three.

4. Don’t Remain in Bed Awake

Number four is walk it out. And what I mean by this is don’t remain in bed awake for too long. And the cutoff is usually about 20 to 25 minutes. If you’ve been trying to fall asleep or you’ve woken up and you’re trying to fall back asleep, and you’re not asleep after about 20 or 25 minutes, get out of bed and go to a different room and in dim light, just kind of read a book or a magazine. And don’t check email, don’t do any of that. And the reason is the following. Your brain is an incredibly associative device. And very quickly it learns that your bed is the place of being awake, not asleep.

And so I hear this from people. They’re saying, “Look, I’m I’m watching television on the couch and I’m falling asleep and then I get into bed and I’m wide awake. And I don’t know why.” And it’s because the brain has learned that association. So you need to break that association and you only return to bed when you are sleepy and there’s no time limit. And that way you will relearn the association that you once, of course, had as a child which is your bedroom is the place of sleep not wake.

Chris Kresser:  So even when people have to get over their anxiety, I’m not sleeping enough and Dr. Walker told me I have to sleep.

Matthew Walker:  That’s right. It’s very natural.

Chris Kresser:  Or so they have to …

Matthew Walker:  You’re setting yourself up, yeah.

Chris Kresser:  … they’ve got to give themselves permission to just reset that for whatever period of time that takes, knowing that it will help them in the long run.

Matthew Walker:  That’s right. Correct. We’re trying to sort of think about the long-term plan here. We have to accept the short-term loss for a long-term gain. So the analogy that I guess there would be, you’d never sit at the dinner table waiting to get hungry. So why would you lay in bed waiting to get sleepy? And the answer is you shouldn’t. You should get out of bed.

5. Limit Caffeine and Alcohol

The final and fifth one usually makes me very unpopular. I mean, I’m generally very unpopular in truth, but this one especially. Which is alcohol and caffeine. Everyone knows, of course, that caffeine can help you wake up, it’s a stimulant. It’s a psychoactive stimulant and actually the only psychoactive stimulant that I know of that we readily give to our children without too much thought. But caffeine is a problem for sleep for a number of ways.

Firstly, of course, it will keep you awake. It’s more difficult to fall asleep and/or stay asleep if you have caffeine in your system. But also, the other interesting part of caffeine is that its half life and its quarter life are probably not well-known by the public. Caffeine has a half life of about six hours. In other words, after about six hours, half of that caffeine is still swirling around in your brain, but it has a quarter life of 12 hours.

So in other words, if you have a coffee at noon, a quarter of that caffeine is still in your brain at midnight. So having a coffee at noon is the equivalent of sort of tucking yourself into bed at midnight and before you turn the light off, you swig a quarter of a cup of Starbucks and you hope for a good night of sleep. And it’s probably not going to happen.

Chris Kresser:  And that’s if you just had one cup.

Matthew Walker:  That’s right. And so that’s one of the issues that the recommendations usually try to cut off about sort of 14 hours before your expected bedtime. The other issue though, some people will say like, “I’m one of those folks who can have an espresso with dinner, and I fall asleep fine, and I just stay asleep.” Even if that’s true, we’ve done these studies where we give you a standard dose of caffeine, that’s 880 milligrams in the evening. Even if you fall asleep and stay asleep, the depth of your deep sleep is nowhere near as deep. In fact, it’s usually about 20 percent less. There’s a 20 percent impairment or reduction in your deep sleep. And to put that in context, I would probably have to age you by about 30 years to get that type of a drop in deep sleep. Or you could do it every night with a cup of coffee with dinner. So that’s caffeine.

Chris Kresser:  Yes.

Matthew Walker:  Alcohol. Alcohol is probably the most misunderstood “sleep aid.” It isn’t a sleep aid at all. Alcohol, again, is a class of drugs in the sedative category. So when you have a nightcap, you’re just knocking the cortex out again. You’re not going into naturalistic sleep. You think you fall asleep faster, but you’re just sedating the brain. Then there’s two other problems with alcohol.

Firstly, it will fragment your sleep. So you wake up many more times throughout the night. And you wake up feeling unrefreshed, but you typically don’t remember those awakenings. So you don’t put two and two together. And the final component of alcohol is that it’s actually very good at blocking your rapid eye movement sleep, your dream sleep. And so again, alcohol is really one of those things to be avoided if you can in the evening.

I typically would offer the non-politically correct good advice as a Britishman, which would be if you are going to drink, just go to the pub in the morning, and that way the alcohol is out of your system by the evening. But I would never say that on a health podcast in public.

Chris Kresser:  Yeah, well, you’re not here to be popular, you’re here to educate us on the importance of sleep and how to get better-quality and more sleep. And I appreciate that. Because as a healthcare practitioner myself, I see so many people that I think are suffering health problems that are at least partially related to sleep deficit. And it’s so easy to look in other places, rather than just looking at the simple, some of the simple things, like diet and sleep, for example.

Matthew Walker:  It’s so true. I think we’re always trying to optimize for these different sorts of physiological systems, how do I optimize my cardiovascular health or my glucose regulation or my cognition. When we fail to realize that sleep, in fact, is the tide that raises all health boats, it’s the superordinate node. And you can almost think of sleep from a health kind of augmenting perspective.

It’s a little bit like one of those big music-mixing decks that you see in these studios, where they’ve got all of the different dials, which are all of the different biological systems. And then there’s that one dial all the way over on the left, which if you move it up, all of the other dials go with it. That is sleep. It’s the Archimedes’ lever. And if you focus on that, you will be doing a remarkable service to your both healthspan and lifespan.

Chris Kresser:  Fantastic point. I love that analogy. So your most recent book, Why We Sleep, was a real tour de force. It was one of my favorite science books I’ve ever read.

Matthew Walker:  Oh, thank you.

Chris Kresser:  So comprehensive and such a great job of pulling together all of the relevant literature and yet explaining it in an accessible way that the lay person can understand. And I think, I highly recommend reading it. Even if it is a little bit alarming for many people, I think sometimes wake up calls are necessary. And this is one that not only for our own health, but particularly for our children’s health and future generations, as you pointed out earlier that we’re, there’s a real moral and ethical issue here to consider, and we can all become our own advocates for sleep and advocates for our children and future generations.

So, Dr. Walker, thank you so much for your pioneering work, your book, Why We Sleep, which is available everywhere that books are sold. I highly recommend that, and I could have easily kept talking to you for a couple hours. I know you have to go. So maybe we’ll have you back on the show in the future at some point.

Matthew Walker:  I would love that, Chris. And Dr. Kresser is what I should call you, but thank you so much for having me here. It’s a message that I’m desperately trying to get out there. I would love to try and help reunite humanity with the sleep that it is so clearly bereft of. So I have to anoint you as a sleep ambassador now for giving me this platform. So, thank you for that.

Chris Kresser:  And you’re welcome. I’ll come down to your center and visit sometime soon, since we’re not so far away.

Matthew Walker:  I would love that. And thank you again for this opportunity.

Chris Kresser:  Take care, thanks again.

Matthew Walker:  Okay, you take care.

Chris Kresser:  Bye.

Matthew Walker:  Bye.