Updated 13th March 2023

The secrets of good sleep

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    We all know how good it feels to drift into deep sleep and wake up feeling refreshed. 

    The positive effects of a good night’s sleep affect every aspect of our lives. We feel energetic, focused, and ready to take on the day’s challenges.  

    But the long-term effects of bad sleep are less known. It turns out, they have a huge impact on our health and even how long we live.  

    In this episode, Jonathan speaks with Prof. Matthew Walker to learn what the latest science says about improving our sleep to give us more energy and better health.

    They also discuss whether how we sleep changes how our bodies respond to food.

    Matthew Walker is a sleep expert, a professor of neuroscience at UC Berkeley, and founder of the Center for Human Sleep Science. He’s also the author of Why We Sleep.

    Matt and his team are currently working with ZOE scientists to research the links between sleep, nutrition, and health.

    If you want to uncover the right foods for your body, head to joinZOE.com/podcast and get 10% off your personalized nutrition program.

    You can discover your chronotype here.

    And you can follow Matt, listen to his podcast, and get his book.

    Transcripts of other ZOE Science & Nutrition episodes are available here.

    And you can also follow ZOE on Instagram.

    This podcast was produced by Fascinate Productions.


    [00:00:00] Jonathan Wolf: So this idea that you know what, if I sleep on this thing, I might feel better in the morning, which is, I think my mother used to always say to me, You know, go to sleep. You'll feel better in the morning.

    [00:00:08] Matt Walker: Correct.

    [00:00:08] Jonathan Wolf: You're saying that was real science. That's extraordinary. 

    [00:00:11] Matt Walker: Yeah. And in fact, there's a wonderful quote by the American entrepreneur, E. Joseph Cossman, once said the best bridge between despair and hope is a good night of sleep.

    [00:00:23] Jonathan Wolf: Welcome to ZOE Science & Nutrition, where world-leading scientists explain how their research can improve your health.

    Thank you for joining me today. I really enjoyed your book, you know, Why We Sleep, which I first read quite a few years ago now when it first came out, and I have to admit, I'm not sure I said this to you before, but I drove my wife crazy, turning our bedroom into a cave afterward. So she definitely knows who you are, even though she hasn't yet met you.

    [00:00:55] Matt Walker: Probably for all the wrong reasons, but at least I anoint you as a sleep ambassador for what you've been doing, Jonathan. 

    [00:01:01] Jonathan Wolf: Well, thank you. I think, as always, it's a bit like with ZOE & Nutrition, you know, I nag at her for so long about this and eventually get into it, and then she becomes more obsessive than me. So, she's on that path with sleep. But anyway, look, it's great to have you here talking about sleep.

    Both your research and I think we're also gonna touch on some of the research that you're working on together with ZOE. Now we have a sort of tradition here on the podcast. We always like to start with a quick-fire round of questions from our listeners, and the rules are really simple. You can say yes or no, or a one-sentence answer, but no more.

    And we know that scientists always find this challenging. So are you ready for me to kick off?

    [00:01:39] Matt Walker: I'm ready for the challenge.

    [00:01:41] Jonathan Wolf: All right, so the first question is can bad sleep kill? 

    [00:01:46] Matt Walker: Yes. In multiple different ways through multiple different diseases. 

    [00:01:51] Jonathan Wolf: I know we're gonna talk more about that. Okay. Is it more important to exercise than to sleep well? 

    [00:01:57] Matt Walker: I would say sleep is the foundation on which exercise and nutrition sit. It's not the third pillar, It's the foundation for those two other things. 

    [00:02:06] Jonathan Wolf: And I'm guessing that the people listening to this cold, waking up at five in the morning to exercise. We'll talk a little bit about whether that's a good idea.

    [00:02:14] Matt Walker: I think we will. 

    [00:02:15] Jonathan Wolf: All right. Is a short nap in the day okay? 

    [00:02:18] Matt Walker: Mostly yes. If you're suffering from insomnia, no. 

    [00:02:22] Jonathan Wolf: Can what we eat affect how we sleep? 

    [00:02:26] Matt Walker: Yes. 

    [00:02:27] Jonathan Wolf: I have a small child, Matt. I get woken up in the night. Is my health doomed? 

    [00:02:32] Matt Walker: No. Simply get sleep whenever you can get it across the day or the night during that time period of ownership of a young one.

    [00:02:45] Jonathan Wolf: All right, That's great advice, which also sometimes is hard to follow. So Matt, what's the biggest myth about sleep that most people still believe? 

    [00:02:53] Matt Walker: There are so many myths, but I think one of the fun myths that have been busted is that counting sheep will help you fall asleep. And there's a great study done here at UC Berkeley, and it wasn't done by me, it was done by a colleague of mine.

    And what they found is that counting sheep not only didn't make you fall asleep any faster, it actually took you longer to fall asleep when you were counting sheep. But what they did find was something interesting. There is an alternative mental strategy. That strategy is taking yourself on a mental walk, so think about a walk that you know really well.

    Maybe it's a walk in the woods or in the forest, or a hike or a walk on the beach. and then try to really visualize that to the point of, this is me leaving my front door, I'm walking down the steps, off I go. And if you do it in granular detail and move through it, the next thing you remember is your alarm going off the next morning because you've fallen asleep and it seems to be quite effective tool.

    So that's one of the many, many myths that we come bust regarding sleep 

    [00:04:07] Jonathan Wolf: Brilliant. Thank you, Matt. And look, why don't we actually just start right at the beginning there about what is sleep? And I know it sounds like a sort of crazy question, right? Because we all sleep. I think it remains sort of one of the more mysterious key activities that happen in a human being.

    So could you just sort of kick off with us there? 

    [00:04:26] Matt Walker: Yeah, it's bloody bizarre, isn't it,? I mean, you know, we close our eyes, and then we think that we essentially lose consciousness and that our body just lies dormant, and then seven to nine hours later we wake up. Now, that is so understandable and if I didn't know what I know about sleep, I would think the same.

    And as a consequence, I perhaps would say, Well, look, what's the big deal of losing 30 minutes or an hour here or there? You know, or just going down to six hours because I'm a busy person, or five and a half hours, because really I'm just missing out on my body, getting some rest, and my mind is not really doing much.

    Nothing further from the truth is the actual fact of it. Your brain is incredibly active during stages of sleep. In fact, during some stages, it's up to 30% more active than when you're awake. 

    [00:05:26] Jonathan Wolf: More than 30% active than when I'm awake. That's crazy! 

    [00:05:30] Matt Walker: Yeah. Some parts of your brain, a 30% more active in some stages of sleep than when you're awake.

    And then downstairs in your body, there is a radical overhaul. There is - it's like hitting the reset button on your wifi router, but it just takes seven to nine hours to do let me just take a step back though, cause I haven't really answered your question. What is sleep? Sleep in human beings, in fact, in all mammalian species is separated into two main types.

    On the one hand, we have something called non-rapid eye movement, sleep, and non-REM sleep. On the other hand, we have rapid eye movement sleep or REM sleep. And REM sleep is the principal stage in which we dream. Now it turns out that non-REM sleep is further subdivided into four separate sub-stages that, unimaginatively called stages one through four, increase in their depth.

    So stages three and four, that a deep sleep that we discussed. And then stages one and two of non-rem, that's light sleep. So you may have seen this in some of your sleep trackers where it says, Were you awake, in light sleep, deep sleep, or REM sleep? 

    Now those two types of sleep, non-REM and REM will end up playing out in a battle for brain domination throughout the night, and that cerebral war is going to be won and lost every 90 minutes.

    And then replayed every 90 minutes creating this standard 90-minute cycling architecture of human sleep on average, in most people. What's interesting, however, is that the balance, the sort of the cocktail mixer distribution of non-REM and REM within those 90-minute cycles, changes as you move across the night.

    What I mean by that is in the first half of your night, the majority of those 90-minute cycles are comprised of lots of deep non-REM sleep and very little REM sleep. But when you push through to the second half of the night, now that shifts and instead you get much more REM sleep and very little deep sleep.

    [00:07:46] Jonathan Wolf: And Matt, what's all of this for? So I mean, we definitely get a picture that it's a lot more complex than I guess most of us imagined, which is, I always sort of felt, Oh, you go to sleep, you wake up. It's sort of annoying, right? Like all those hours when you could do something better. I think you're starting to paint this picture of a lot of complexity.

    Why is any of this happening? 

    [00:08:06] Matt Walker: Learning memory, emotional brain regulation, brain plasticity downstairs in the body and overhaul of your cardiovascular system, a replenishing of your immune system, a reregulation of all of your hormonal systems. And in fact, 50 years ago, we used to ask the question, Why do we sleep?

    And the crass answer at the time was that we sleep to cure sleepiness, which tells you nothing about, you know, the meaningfulness of sleep. Now, 50 years later, we've had to upend the question. We now have to ask, is there any major operation of your brain or is there any major physiological system in your body that isn't wonderfully enhanced when you get to sleep or demonstrably impaired when you don't get enough?

    And so far the answer seems to be no. 

    [00:09:00] Jonathan Wolf: That's amazing. And so how does that tie in? You're describing these different stages. Are they linked to particular elements of the way in which sleep is sort of creating all of these benefits for us?

    [00:09:11] Matt Walker: Very much so. So what we've learned is that all of those stages, even some of the light forms of non-REM sleep, All of those stages of sleep are important.

    And so sometimes people will come up to me and say, How do I get more deep sleep? Or How do I get more REM sleep? And my question to them usually is, Why do you want to get more REM sleep or deep sleep? And they'll say, Well, isn't that the good stuff,? And it turns out, and it's all good stuff, it's just that different stages of sleep will do different things for your brain and your body.

    At different times of night and we can't shortchange the brain on any one of those stages without suffering some kind of deleterious impairment. 

    [00:09:57] Jonathan Wolf: So you need all of these different stages that you're describing because they're each doing different things for both our mind and sort of the rest of our body.

    Is that what you're saying? 

    [00:10:06] Matt Walker: Correct. And when you think about it from an evolutionary perspective, of course, that must be the case because when we're asleep - sleep is the most idiotic of all, you know, inventions because when you're asleep, you're not finding a mate, you're not finding food, you're not reproducing, you're not caring for your young and worse, you're vulnerable to predation.

    [00:10:24] Jonathan Wolf: Yeah, no, I hadn't really thought about it that much, but you're right. Of course, being asleep is a terrible place to be, isn't it? And Matt, do we understand what is going on in this REM stage? 

    [00:10:33] Matt Walker: We do. REM sleep is associated with dreaming, but dreaming may just be an epiphenomenon. You could imagine that dreaming is like a light bulb.

    The reason that we create the apparatus of a light bulb is to create light, but it turns out that when you produce light in that way, you also produce heat. It was never the reason for the light bulb. It's just what happens when you produce light in that way. And the same could be true for dreaming the reason that the brain created this thing called REM sleep was to serve lots of different functions.

    But when you do create REM sleep in the way that we have, you also get this byproduct called dreaming. It's just an epiphenomenon. Well, now we know that REM sleep and dreaming actually are all functional. The things that we've discovered are at least twofold.

    The first is that REM sleep provides a form of overnight therapy. It's emotional first aid, and it's during dream sleep and the particular brain chemistry of dream sleep, where we reactivate emotional experiences and events that we've had during the day or even in past years or months and we, sort of process that information. And in doing so, it is almost as though REM sleep is a nocturnal soothing bomb that takes the sharp edges off those difficult, painful experiences.

    So you wake up the next day and you feel better about them. So, in other words, it's not time that heals all wounds, but it's time during dream sleep that provides emotional convalescence. 

    [00:12:14] Jonathan Wolf: That's amazing. So this idea that you know what, if I sleep on this thing, I might feel better in the morning, which is, I think my mother used to always say to me, you know, go to sleep.

    You'll feel better in the morning. 

    [00:12:23] Matt Walker: Correct. 

    [00:12:24] Jonathan Wolf: You're saying that was real science. That's extraordinary! 

    [00:12:27] Matt Walker: Yeah, and in fact, there's a wonderful quote by the American entrepreneur, E Joseph Cossman, who once said, the best bridge between despair and hope is a good night of sleep. That's exactly what we found. So that's one function of dreaming, of REM sleep.

    The second function, however, relates to your learning and memory abilities. And what we found is that deep non-REM sleep is the stage of sleep where we essentially cement new memories into the brain. It's like hitting the save button on that document, but when we do that, for all of the information that we've been learning is during deep sleep, and it takes a little bit longer than hitting the safe button, but what does REM sleep do then?

    Well, it turns out that REM sleep is a form of almost informational alchemy. That REM sleep then takes all of the information that you've recently learned and starts colliding it and interconnecting it with all of your past back catalog of experience and creates new and novel links and associations so that you wake up the next morning with a revised mind wide web of associations that is now capable of defining solutions to previously impenetrable problems. 

    [00:13:48] Jonathan Wolf: So this is the sort of eureka moment when you wake up the next day and suddenly, you know, it all sort of makes sense, even though yesterday, you know, you couldn't really figure out the solution to whatever the problem was.

    [00:14:00] Matt Walker: That's exactly right. I mean, think about it. No one has ever told you, Jonathan, that you should really stay awake on a problem, you know? Instead, they tell you to sleep on a problem. 

    [00:14:12] Jonathan Wolf: It's amazing and I think we're going to talk a bit more about the impact of good and bad sleep. Just before we move off this topic, because, you know, I think I could keep asking you questions about exactly how sleep works forever, but how do you figure this out in your lab?

    You know, how do you actually understand what's going on given that, you know, this is very different from many of the topics that we look at here at ZOE, where, you know, you're constantly measuring blood, for example. You can see the changes, but how are we able to get these sorts of deep insights that you're talking about?

    [00:14:39] Matt Walker: We use a vast array of different techniques. My sleep center here at UC Berkeley. So we will measure your sleep with what's called high-density EEG, which means I'm going to stick hundreds of electrodes all over your head. You look like this spaghetti monster. And that allows us to pick up all of the special brainwaves that are going on during the different stages of sleep.

    [00:15:02] Jonathan Wolf: And do you have to shave my hair to do that or am I allowed to keep that on? 

    [00:15:06] Matt Walker: You would think, but no, we don't, thankfully. 

    [00:15:09] Jonathan Wolf: Oh, that's, that's good.

    [00:15:10] Matt Walker: It's somewhat easy to do. 

    [00:15:12] Jonathan Wolf: All right, then I'm signed up. 

    [00:15:14] Matt Walker: So yeah, don't worry, you'll keep your wonderful hair and, but then we also use a whole variety of other technologies.

    We use MRI, particularly to look deep into the brain and look at the patterns of brain activity that are changing when you are in and out of different stages of sleep. We also look at how your brain has changed before and after a night of sleep, we use special PET scans in a lot of our work, which I think we'll discuss on aging and dementia, to look at different Alzheimer's disease pathology buildup because of insufficient sleep.

    And then we use lots of the peripheral markers that you've been describing. We measure blood. So we look at lots of inflammatory factors, for example, in blood. We also measure other aspects. We measure your cardiovascular system. We measure hormonal systems. We measure your Thermo regulatory system as well.

    And at this stage, there's probably almost not one major system within your body that we're not really measuring in the laboratory. 

    [00:16:18] Jonathan Wolf: And so I think that one of the things you are really passionate about is what happens when people don't sleep enough, right, Matt? 

    [00:16:24] Matt Walker: Yeah. 

    [00:16:24] Jonathan Wolf: So can you talk a bit about this?

    Because I think we do live in this world where, because of electric light, because of digital devices, we no longer sort of run out of anything interesting to do after it gets dark and therefore probably, you know, clearly just sleep as much as our body wants. Most of us have to work quite hard, in fact, to get as much sleep as I think we would naturally do, apart from maybe my teenage son.

    He's just fine if I leave him alone. So what happens to people if they don't sleep enough? 

    [00:16:54] Matt Walker: So downstairs in the body, we know that short sleep or insufficient sleep will change your cardiovascular system for the worse. It will increase your blood pressure. It will increase the speeding contraction of your heart and it will reduce - which is not a good thing - something called your heart rate variability. And so firstly, we see significant impacts on your cardiovascular system, and this is the reason why short sleep across the lifespan increases a whole collection of cardiovascular disease features, things such as atherosclerosis. And we published a paper on this recently that having just poor quality sleep and fragmented sleep increases inflammation.

    And that inflammation then leads to the buildup of plaques in your arteries and that leads to cardiovascular disease. The next thing we could speak about is the immune system. There is a very intimate association between your sleep health and your immune health. For example, we know that individuals who - we've seen some of these experiments as well - if you limit someone to just four hours of sleep, for one single night, there is a 70% drop in critical anti-cancer fighting immune cells called natural killer cells.

    And so that's, you know, quite a concerning state of immune deficiency after one short night of sleep. 

    [00:18:22] Jonathan Wolf: And that's just one night of short sleep. 

    [00:18:25] Matt Walker: Yeah, and we also know that, for example, and we've just got the data through for Covid, in the same is true here, but if you are not getting sufficient sleep in the week before you get your flu shot, you will only produce 50% of the normal antibody response.

    Therefore, rendering that vaccination, you know, far less effective if not, not effective at all. I think the other big finding that's burst onto the scene, and I think it's probably the most exciting finding recently in sleep science, is the link between a lack of sleep and Alzheimer's disease. And we do a large amount of this.

    We have multiple large research programs looking at this, at my sleep center. What we first understood is that people who don't sleep enough, who sleep six hours or less, people with insomnia, or people with a sleep disorder called sleep apnea, all of those people have a significantly higher risk of developing Alzheimer's disease in later life.

    Now, the problem is that that's just correlational. That's not causal. Correlation went in search of causation, and we found the evidence both in animal studies and in humans. For example, if you take a healthy human and you deprive them, you selectively deprive them of either just deep sleep or you deprive them of sleep for an entire night, the next day we see an immediate buildup in the toxic proteins associated with Alzheimer's disease called beta-amyloid and tau protein, and we see that circulating in your bloodstream, circulating in your cerebra spinal fluid and using special brain scans, we've been able to see that build up in your brain itself.

    So that was a causal demonstration that a lack of sleep is associated with a buildup of Alzheimer's protein. Reverse that question though. If a lack of sleep increases your Alzheimer's disease proteins, then what is it about sleep when you get it that deescalates those proteins and de-risks you from the disease of Alzheimer's?

    Well, we then subsequently find, and this was a finding by Maiken Nedergaard at the University of Rochester. She found that the brain has a cleansing system. Now, we knew that the body had one called the lymphatic system, but we actually didn't think the brain had its own cleansing system, but it does. But it's called the glymphatic system named after the glial cells that make it up.

    But then we made more discoveries, which is that that cleansing system in the brain, that sewage system essentially isn't always switched on in high flow volume during the 24-hour period. In fact, it's only by drawing sleep and specifically drawing deep sleep that the brain switches on that cleansing system and washes away all of the metabolic detritus that has been building up during the day. And it's crass. And it's hyperbolic. But you could say certainly from a biochemical perspective, wakefulness is low-level brain damage. And what we've now discovered is that sleep is your sanitary salvation. And so it's a good night, sleep, clean, as it were. It's a power cleanse for the brain. 

    [00:21:42] Jonathan Wolf: There are definitely some interesting analogies elsewhere, right? Like, so I guess if you think about muscles, we all know, I guess from people talk about training that, you know, actually the act of using muscles causes damage and that the rest and recovery is central. We discuss that you know, areas that I spend more time on around the gut, like how important it's now become clear that you need to leave long periods of time for the gut also to recover.

    And that's partly to do with the microbiome but partly to do with these processes. So it's really interesting that across all these different areas of the body, you're seeing the same thing in the brain, and that does sound a bit like my mother's saying, Well, you need to have a rest. Right? You need to let, your mind rest. It's funny also, how some of these things feel very similar to the sort of traditional advice that our parents and grandparents would've given us, which maybe have become harder to follow now that we have all these great devices that can keep us up all night. 

    [00:22:31] Matt Walker: Yeah. I often say that really all I do as a sleep researcher of the past 20 or so years is put the science behind everything that your mother ever told you about sleep, so... 

    [00:22:42] Jonathan Wolf: Well, I've always believed that doing what your mother tells you is a good idea, even if I don't always actually achieve it. So, you know, that's really interesting. 

    [00:22:48] Matt Walker: Yeah. But let me come back to the stuff that we're doing together, I mean, it's incredibly exciting because, with a lot of the studies that we and other colleagues have done in the field when we measure your sleep, we typically will measure your sleep for just a night in the laboratory, because it's difficult to constantly track individual humans, you know, night after night after night.

    It's especially difficult to track large numbers of them from one night to the next, to the next, to the next, and then it's even harder in addition, be measuring lots of changes in their brain and their body as a consequence of that ongoing night to night sleep evaluation, meaning that we know a lot what we call cross-sectionally, so we just take a large group of people, we do one night of sleep recording, we measure changes the next day and we show that there are these associations.

    What that doesn't really tell you however is what is the consequence within an individual of variability in their sleep across weeks, if not months? And that's a fundamental question cause that's the way most of us live our lives. So what I'm saying is that we've not really understood what inter-individual differences are within an individual over time as their sleep fluctuates.

    What can we learn about that? And how does that relate to things such as the metabolic system and the immune system and the gut microbiome? That's the type of work that we're able to do in the ZOE collaboration.

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    And then we're also able to look at genetics because of course, as you've mentioned before on the podcast, there is a remarkable set of twins within the large data set, which gives us the ability to look at lots of different things, but one of them is genetic heritability and so that is very powerful too. And, and I can mention if you'd, like one finding that we're soon going to be publishing, one of the most fundamental questions that we wanted to ask was the following. How will you wake up tomorrow morning?

    And secondly, how will you sustain that wakefulness across the day? What is it about your sleep or your food or your genes that determine how well you wake up every day and how able you are to stay awake? And you'd think that we would know the answer to that question. We didn't, and we were able to answer it with the data set.

    And so, Looking at over a thousand healthy individuals, which also contained, a large number of both identical and fraternal twins. And what we found is that how well you wake up the next day has nothing to do with how much sleep you get relative to the standard population recommended average. Instead, it was how if you slept longer relative to your own typical average amount of sleep, then you wake up more effectively. It takes you a shorter amount of time to wake up.

    The engine heats up quicker in terms of operating temperature, which is what we call overcoming sleep inertia, and then you are far better at sustaining that wakefulness across the day if you've slept longer than is your normal.

    The second part is that we found it's not just about your sleep, it's also about your food and expressly your breakfast. And what we find is that breakfasts contain slightly higher amounts of fat and fiber, but low amounts of simple sugar. That type of breakfast predicted a far superior ability to wake up and then stay awake across the day.

    The next thing we found, and you would think that is just the innate levels of how alert someone is. You know, you can find some people who just seem to have crazy energy, you know, each and every day, and other people who are just a bit more sort of baseline relaxed. You would think that that means that it's strongly genetic.

    Well, we did a twin pair analysis and we demonstrated that using this data set, genetics only have a very small contribution in determining how well you can wake up. And stay awake throughout the day. And I think that that's actually very encouraging because it means that you are not bound to a predetermined genetic fate that most of how well you feel when you wake up every day and stay awake across the day is determined by non-genetic, in other words, modifiable, influential factors. 

    [00:27:41] Jonathan Wolf: I think that is incredibly exciting, right? Both this idea that it's personalized, it's not just one answer for everybody, but people are different. But also I think we see this across a lot of our research that you're less locked in by your genes. And I think many of us were led to believe over the last sort of 40 or so years. And I think that's really reassuring, right? Because it suggests there are still things you can do to change this. 

    [00:28:03] Matt Walker: So much so. That's exactly the message that how you sleep and how you eat each and every day will decide how well you wake up and feel awake each and every day. 

    [00:28:16] Jonathan Wolf: And do you see changes, you know, across time?

    And I think one of the topics that, you know, we had a lot of questions from our listeners about was around menopause, which is obviously a really big change that we touch on in a lot of places. Is that something that's well-studied and what's going on there? 

    [00:28:32] Matt Walker: Personally, I don't think it's studied anywhere near enough, but we know that there are marked changes during that, what we call the sort of the perimenopausal period.

    So during menopause and both before and after it, there are really significant increases in sleep disturbance, and sleep becomes more fragmented. When you wake up, people will find it harder to fall back asleep. And then there are the temperature swings, the temperature changes, these hot flashes that people will experience.

    Why is that a problem? Well, it's a problem at night because it turns out your brain and your body need to drop their core temperature by about one degree Celsius for about two to three degrees Fahrenheit, for you to fall asleep and stay asleep. And that's the reason you will always find it easier to fall asleep in a room that's too cold than too hot because the room that's too cold is taking you in the right temperature direction for good sleep.

    But if you're having these changes inside of the body and the brain that create a thermogenic effect that creates these bursts of heat that we call these hot flashes, no wonder you're struggling to stay asleep. So menopause is certainly one of those time periods where we see sleep disruption. And then also just as we get older, unfortunately.

    We know that probably one of the cognitive hallmarks of getting older, of course, is that our learning and memory abilities begin to fade and decline, and that's been known for a long time. What we've also discovered though, is that one of the most reliable physiological signatures of aging, Is that your sleep gets worse, and not just any type of sleep, by the way.

    It's particularly that deep quality of sleep that we were describing before, which is actually essential for some of the cardiovascular functions, for some of the learning and memory functions for some of the immune system functions, all of which change with aging. That's the type of sleep that declines most precipitously as we get older.

    And the worrying thing is, by the way, that great sleep depression for deep sleep, at least, we can start to measure it happening in your mid to late thirties, which is exciting for me because that means that's an opportunity for prevention. I think, you know, this is one of the goals of the ZOE project too, of course. Is that we have a model of sick care and we don't have a model of healthcare, and we don't really understand the question of, rather than what happens when a human being goes wrong, how do you treat that disease? How before you ever get to that stage, do you maintain a human in the right healthy direction?

    And that is a model of healthcare rather than sick care, which is what we have now in modern societies. 

    [00:31:28] Jonathan Wolf: And I think, you know, this is something I, you know, I believe in passionately and you know, it's so clear that sleep is one of these central pillars, right? That if you get right can really improve your long-term health.

    And I guess, I stopped you mid-flow there, Matt, but I just say yes, we're incredibly excited. Obviously, we think nutrition is a very important part of this, but ultimately, This is about understanding, you know, the set of things that you can do before you're really sick to try and make sure actually you put that off for, for as long as possible.

    [00:32:02] Matt Walker: That's exactly right. And that's why I think this, you know, in some ways, it's depressing to know that it's in the mail for all of us. You know, I'm now coming solidly into the foothills of the middle-aged myself and, I've already seen the changes in my sleep, I can notice them. But what if we can do things that could intervene and actually prevent that decline of deep sleep?

    Could we, for example, even bend the arrow of Alzheimer's disease risk down on itself by way of superior sleep intervention in midlife? That's one of the things that we're very excited about trying to do at my sleep center. 

    [00:32:38] Jonathan Wolf: And I think it's a brilliant transition to the last topic. And I would say just personally, I have really noticed I'm in, in my late forties now that I do not sleep as well as I used to. I used to be someone, I would've considered a really good sleeper. And there's no doubt that I'm more easily disturbed when I'm disturbed I find it harder to go back to sleep. And this is even after using all of your great tips. But shall we transition to that?

    Because we always like to make sure we give actionable advice. And I know you have some really great advice for people listening to this who are now saying, You know what? Maybe I didn't take sleep as seriously as I should have done. Now I wanna take it really seriously. How can I go and get better sleep?

    What would your advice be, Matt? 

    [00:33:17] Matt Walker: Oh, I think there's a couple of general tips, and you can find most of these on the internet too, but it's good to go over them. The first thing is regularity. I would say if you could just focus on one thing, go to bed at the same time, wake up at the same time, no matter whether it's the weekday or the weekend, and I'm pretty religious like this, you, you know, and not because I wanted to be a poster child for good sleep.

    It's selfish, you know, if you knew everything I knew about sleep and how important it is, you wouldn't do anything different than prioritize your sleep. And I do. So I would say make sure you're getting somewhere between a seven to nine-hour opportunity. Find out what is innately correct for you, which is called your chronotype.

    Are you a morning type, evening type, or somewhere in between you need to sleep in harmony with your chronotype to get the best sleep? So, I'm somewhere in the middle, just like the rest of my personality. I'm quite vanilla in terms of my chronotype. So you're either a morning type, evening type, or neutral.

    And I'm mostly neutral. I'm kind of somewhere between 11 to 11:30, kind of, you know, 7:30, 7:45 wake-up time, which puts me in the neutral category. If I were to go to bed, you know, at 9:00 PM. And then wake up eight hours later, or I was to go to bed at 4:00 AM and wake up eight hours later versus my natural eight-hour sleep window. Well, it's eight hours. So what's the difference that there's sure there's no difference? Well, there's a big difference because. In one of those three scenarios, I will have been sleeping in sync with what my biological rhythms want me to do. And the other times I will be out of sync and I won't sleep as well.

    But the first message is regularity. 

    [00:35:07] Jonathan Wolf: And Matt, just question because I am, I know that we'll get a flurry of these questions afterward. How do you find out your chronotypes so that you know that you can be sleeping in line with that? 

    [00:35:18] Matt Walker: If you want to do the detailed assessment, you can go onto Google and you can search for something called the M-E-Q, which stands for morningness, eveningness questionnaire takes you about three or four minutes to fill out, and then you'll get a score and that score will tell you, in fact, we really use in sleep science five categories, extreme morning type, morning type, neutral, evening type, extreme evening type, and it will kind of bucket you into one of these flavors.

    The other way you could really do it though is kind of like my quick rule of thumb, and it's just a rule of thumb. It's hardly a rule. Let me ask you the following question. If you're on a desert island, nothing to wake up for. No pressures, no one to wake up for, no work. What time do you think you would like to go to bed? And what time do you think you would like to wake up? And the answer to that question is usually very different than currently when you have to go to bed and when you have to wake up. And that mismatch is the misalignment between how you are forced to sleep versus how you are biologically designed to sleep.

    So that's another way that you can sort of answer the question. And it's relevant by the way. Some people come to me and say, I've got vicious insomnia. I get into bed and I cannot fall asleep for the first hour or hour and a half. And then we go through this exercise of figuring out their chronotype. And what you realize is that they're going to bed at 10:00 PM because they have to wake up at six to go to work.

    And in fact, they're much more of an evening type they would normally like to go to bed at, maybe 12, 12:30. And so they don't necessarily have insomnia. They have this mismatch between their chronotype and when they start sleeping a little bit closer to their natural sweet spot, they sleep better. So it is relevant to know your chronotype.

    [00:37:09] Jonathan Wolf: That's fantastic. And we will provide links in the show notes to the questionnaire for anyone who's listening to this and make sure that it's the right one. 

    [00:37:16] Matt Walker: Yeah. And then other than that, I would just probably quickly go through a few others. The temperature we've already spoken about keeps your bedroom cool.

    Aim for around about 65 to 67 degrees Fahrenheit or around about, what is that, 18, 18.4 ish degrees Celsius. I know it sounds cold, but cold it must be. You can keep your feet warm, with hot water bottles, and socks, that's fine. But the ambient must be cold. 

    [00:37:43] Jonathan Wolf: And that's colder than many people keep their rooms today, right?

    [00:37:47] Matt Walker: It is, yeah. Most people will come home, they'll set, you know, have this ambient temperature of, you know, 70, 72 degrees in their house, and then they leave that same thermostat setting for the night and we need to cool down at night. So keep the temperature in mind. Light is another thing. We are a dark-deprived society in our modern era, and we need darkness at night to trigger the release of a sleep hormone called melatonin.

    And so, as a tip, I don't like the word hack, but as a tip, try doing the following experiment. In the last hour before your bed, so set an alarm on your phone or on your home device, say, set an alarm for whatever time it is now before bed, and in that last hour, dimmed down half of the lights, even more of the lights if you can, dim down half of the lights in all of your houses and you will be surprised at how sleepy that increased darkness will make you feel. And what that tells you is that normally you are suppressing the release of this sleepiness hormone melatonin, or it's a sleep timing hormone really, when you are bathed in electric light at night.

    So light is another one. The final two things I would mention, are alcohol and caffeine. I know, I know. I'm sorry. This is bad news. 

    [00:39:10] Jonathan Wolf: Yeah, you are a bit depressing on this one, Matt, I have to say. But I think you should tell everybody. So that they get all the facts. 

    [00:39:16] Matt Walker: Well, I'm much more bullish now. You know some people, sometimes on different podcasts when I'm interviewed will say, you know, What have you changed your mind on in the last five years, I've changed my tune on coffee. I would say drink coffee because the health benefits that have been associated with coffee are profound and very reliable. But here, when it comes to sleep, the dose and the timing make the poison. By the way, the reason that coffee is associated with health benefits has nothing to do with caffeine.

    The reason is that the coffee bean contains a whopping dose of antioxidants. And because most people, and you know this better than most of us, Jonathan, most people in the western world are deficient in their dietary intake. And the way that most people get their daily dose of antioxidants is through their cups of coffee, and that's why coffee is associated with health benefits.

    Case in point, you get very similar health benefits from decaffeinated coffee. So it's not the caffeine, it's the coffee bean itself, but I would say the dose and the timing make the poison. Try to limit yourself to two cups on average, maybe three, but the critical thing is to cut yourself off at least 12 hours before you expect to go to bed.

    That's a good rule of thumb.

    [00:40:38] Jonathan Wolf: And I would say just on that one, I, I'm actually more of a tea addict than coffee. But I have definitely discovered that this timing with the caffeine is important and I now cut myself off. I think this is something you could sort of figure out for yourself, Matt, a bit, right?

    Cause there's a lot of personal variation in caffeine response, isn't there? 

    [00:40:56] Matt Walker: Yeah, there is. And we know the genes that change the clearance, the speed of clearance, and caffeine. 

    [00:41:00] Jonathan Wolf: But in my case, that means I need to stop by about two or three o'clock in the afternoon. And if I go later, then, sure enough, you know, it affects my sleep.

    And sometimes also, it can even mean you wake up again right in the night, and then you can't go back to sleep. So it's a bit more complicated than I had imagined. 

    [00:41:17] Matt Walker: Both make it harder to fall asleep, then it fragments your sleep. But the other thing that's pernicious about caffeine, some people will say, Look, I'm one of those individuals, and they could be because they clear caffeine very quickly but not quickly enough as we'll see. They'll say, I can have an espresso with dinner and I fall asleep and I stay asleep, and I'm just fine.

    Even if that's true. Caffeine can actually decrease the amount of deep sleep that you have by somewhere between 12 to 15%. It depends on the dose of caffeine. Now, to reduce your deep sleep by 15%, I would have to age you by about, you know, 10 to 12 years. Or you can just do it every night with an espresso, with dinner.

    So it's a little bit... You know, be thoughtful. 

    [00:42:04] Jonathan Wolf: What about alcohol? 

    [00:42:06] Matt Walker: Yeah. You know, many people when they're struggling with sleep will turn to alcohol as a quote-unquote sleep aid. Unfortunately, it is anything but a sleep aid. Alcohol is in a class of drugs that we call sedatives and sedation is not sleeping.

    So when you have a couple of nightcaps, people say, Look, I always fall asleep faster if I've had a few drinks in the evening. You're not really falling asleep faster, you're just losing consciousness more quickly. And that's the first problem. Alcohol is a sedative. The second is that alcohol will fragment your sleep like caffeine but through a different chemical mechanism.

    So you wake up many more times throughout the night, but the problem is you typically don't remember those awakenings. So the next morning you wake up, and you feel unrestored and unrefreshed, but you don't remember waking up. And so you don't put two and two together.

    The final reason that alcohol is not great for your sleep is that it is quite potent at suppressing your REM sleep or your dream sleep. And we know that dream sleep, as we've spoken about, has lots of benefits for the brain. It's critical for the body too. REM sleep is the peak time during the 24-hour period when men and women release their peak levels of testosterone, for example.

    So we need REM sleep. I'm very nervous as a scientist to tell anyone how to live their life. I don't think I have any business doing that. What I'm here to do as a scientist is simply impart knowledge so that you can then make an informed choice as to how you want to live your life. And of course, my goodness, when it comes to cups of coffee and you know, having a drink now and again, life is to be lived for goodness sake.

    So don't get puritanical about it, but just know the evidence and know that there can be consequences. By the way, I would say that with alcohol, the politically incorrect advice that I would never offer you would be to go to the pub in the morning, and that way the alcohol is out of your system by the evening, and then you'll be just fine. But I would never say such a thing on a health podcast. 

    [00:44:14] Jonathan Wolf: And we did a whole podcast on alcohol and I think it's a very interesting topic, but I think the impact on sleep is clearly one of the big downsides. And Matt, one thing that you haven't mentioned, but it was so influential on me, was about making your room like a cave so you don't get woken up in the morning.

    Is that something that's still, believe is really important?

    [00:44:33] Matt Walker: Yeah, very much so. This is temperature, so it's not just about temperature, but it also combines the third tip I've mentioned, which is darkness. So, keep your bedroom cool and dark and then you know, if you need to, you can use earplugs or a sound machine.

    We don't know too much about sound machines, whether they're helpful or helpful to sleep right now. I think for the most part they seem to be mostly benign. 

    [00:44:57] Jonathan Wolf: Just before we wrap it up, I think one thing I realize you haven't mentioned is screens, and that comes up quite a lot. 

    [00:45:04] Matt Walker: Yeah, it's a good question.

    You know, this comes to the light issue again, particularly light exposure at night. And unfortunately, our screens are enriched in the blue LED light spectrum, which is the worst for our melatonin levels. It suppresses it most powerfully. I would say that, in probably over 50 to 60% of the studies looking at blue light screens, they have an impact on sleep, but some of them have not found a robust effect.

    What we do now know is that those devices, perhaps the greater detrimental impact on sleep is not necessarily the light, but they're activating, engagement. Because when you are on these devices, particularly your phone, it is designed to capture your attention, make you alert, and keep you awake and sustained, and engaged.

    And many people will be what we call sleep procrastinating, where they are perfectly tired, but they're so engaged with their device that they can't put it down. That seems to be, if anything, It's this alertness that actually masks otherwise very strong sleepiness. So my rule of thumb, again, not to get puritanical about it, is that the genie of technology is out of the bottle and it's not going back in anytime soon, no matter what I say.

    So, you know, use your phones and your screens. Just keep in mind that they can have an impact on your sleep. And the rule of thumb, I much prefer people to keep their phones out of their bedrooms. If you absolutely have to take it into the bedroom, here's the rule, you can only use it in the bedroom standing up.

    [00:46:45] Jonathan Wolf: I hadn't heard that one before! 

    [00:46:46] Matt Walker: Yeah, it's, it's really interesting. You kind of think, Okay, after about five or six minutes, I'm just gonna sit down on the bed. No. At that point, that's the rule you've done. Put the phone away. 

    [00:46:55] Jonathan Wolf: That's brilliant. And, I have to say, keeping it out of the bedroom is one of the things that I've become quite strict about, but it's not an easy thing always to carry out. I would like to just quickly wrap up as we always do and make sure to summarize what's been, as always, a very wide-ranging conversation. So I think firstly, you know, the big message is sleep is incredibly important. You need your sort of seven to nine hours, whatever it is for you.

    And if you don't, it has these really profound health impacts. We're taught a little bit about the studies. We've been collaborating together on this really exciting new paper that's been coming out shortly about how we wake up every day. How are you able to stay awake about how different each of us is?

    We talked a little bit about sleep disturbance around menopause. You gave us a bunch of tips. Regularity, finding out your chronotype, getting your room colder. Finding a way to get your room a lot darker, both when you go to sleep and when you wake up. And then I think you've got a little softer on caffeine and alcohol than perhaps when I read the book a few years ago.

    But in general, coffee's probably fine, but make sure it's long before you go to sleep. Alcohol, I think you're basically saying, you know, it's, it's never probably really good for you. But bear that in mind. And in terms of screens, if you are gonna bring them into the bedroom, you have to stand up. 

    [00:48:13] Matt Walker: Great summary.

    [00:48:14] Jonathan Wolf: Matt, it was a real pleasure and we look forward to talking again in the future, I hope with the next paper that we bring out. 

    [00:48:19] Matt Walker: Sounds good. Take care. Thank you for hosting me, Jonathan. I really appreciate it. And thanks for the collaboration that we have. I think it's immensely powerful and it's a privileged data set, so thanks very much.

    [00:48:31] Jonathan Wolf: You're welcome. Thank you, Matt. Bye bye. 

    [00:48:33] Matt Walker: Bye. 

    [00:48:35] Jonathan Wolf: Thank you to Matt Walker for joining me on ZOE's Science and Nutrition today. We hope you enjoy today's episode. If you did, please be sure to subscribe and leave us a review. We do read all the feedback. If this episode left you with questions, please send them in on Instagram or Facebook and we'll try to answer them in a future episode.

    At ZOE, we want to improve the health of millions by understanding the right food for each of us, improving our health, and managing our weight. Each member starts with an at-home test, comparing them with participants in the world's largest nutrition science study. If you're interested in learning more about ZOE, you can head to joinZOE.com/podcast and get 10% off your personalized nutrition program.

    As always, I'm your host, Jonathan Wolf. ZOE Science and Nutrition are produced by Fascinate Productions with support from Sharon Fedder, Yella Hewings-Martin, and Alex Jones here at ZOE. See you next time.

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