I read this book (by a sleep scientist called Matthew Walker) because I knew that it would tell me to sleep more, and I hoped it would cite enough scary statistics that I'd be likely to actually follow through. Well, it worked - I'm keeping a copy on my bedside table for the foreseeable future, just as a reminder. In addition to the exhortations to get more sleep, it contains a variety of other interesting and important facts about sleep.

What is sleep?

  • Human sleep consists of cycles lasting about 1.5 hours, each of which contains first a period of NREM (Non-Rapid Eye Movement) sleep, then a period of REM sleep. In brain scans, the former consists of slow, deep brain waves, while the latter shows the same frenetic activity as an awake brain. As the night goes on, cycles feature a higher proportion of REM sleep. This means that if you cut your sleep short by 25%, you're actually missing out on somewhere between 60% and 90% of REM sleep. 
  • REM sleep is when the majority of dreams happen. While it's uncommon for dreams to replay events from our everyday lives, they do often reflect our emotional preoccupations. To prevent ourselves from flailing around during dreams, we enter a state of sleep paralysis, where our brains are unable to control our voluntary muscles. Eyes are an exception - hence the name REM. It's definitely not true that REM is the only valuable type of sleep - in fact, immediately after sleep deprivation the brain prioritises catching up on NREM.
  • The slow waves of NREM sleep are useful for transferring memories from one part of the brain to the other - in particular, from short- to long-term storage.
  • Walker's theory is that NREM sleep is used to prune away unnecessary connections, while REM reinforces useful connections. He uses the analogy of a sculptor who alternates between carving away whole chunks of marble (NREM) and then adding fine detail on whatever's left (REM). From this perspective, it makes sense that REM sleep is concentrated in later cycles. However, it's unclear whether this is the scientific consensus.
  • There are two systems controlling sleep and wakefulness. The circadian system follows the day/night cycle, making you tired in the evening and alert in the morning (the exact timings vary by person, making some people "night owls" and some "morning larks"). In addition, "sleep pressure" is controlled by adenosine, which builds up while you're awake and is cleared away during sleep. Caffeine works by temporarily blocking adenosine receptors, but doesn't prevent it from continuing to build up.

What's it good for?

  • There's a very strong link between NREM sleep and memory. The formation of long-term memories suffers if we don't get enough sleep (even several days after the events we want to remember). This is true both for memories about facts and experiences and for "muscle memory" of actions like playing an instrument. When sleep-deprived, we also have worse short-term memory.
  • REM sleep is important in emotional regulation and creativity. After sleep deprivation, the responses of the amygdala (responsible for strong emotions) can be amplified by over 60%, due to weakened links between it and the prefrontal cortex (responsible for "rational" decision-making). Dreams during REM sleep allow us to make unusual and creative connections between different topics - many great intellectuals report that their best ideas just "came to them" upon waking.
  • Sleep deprivation massively reduces our ability to concentrate. In addition to slower reaction times, when tired we lapse into "micro-sleeps" during which we're totally unresponsive. Walker emphasises that tiredness is a far bigger cause of traffic accidents than drunk-driving, that drivers systematically underestimate how tired they are, and that drivers who micro-sleep often don't brake at all before collisions.
  • In the long term, sleep deprivation increases the risk of Alzheimer's (since toxins are flushed from the brain during sleep), heart attacks (by provoking a stress response from the sympathetic nervous system and raising blood pressure) and cancer (by devastating the immune system). All of these seem to be very big effects - e.g. sleep-deprived patients are twice to three times as likely to suffer calcification of their coronary arteries.
  • Note that most of the effects above are noticeable even after small amounts of sleep deprivation, like getting one or two hours less sleep for one or two nights. In fact, even the one-hour sleep reduction from Daylight Savings Time causes a spike in heart attacks.
  • Sleep is also linked to many mental illnesses - e.g sleep deprivation triggers mania or depression in bipolar patients. Most mental illnesses disrupt sleep, which exacerbates their other negative effects.
  • REM sleep promotes the formation of neural links in infants, who have far more neural connections than adults. It is also important for their language learning.
  • Walker's broad answer to the question of what sleep is useful for: EVERYTHING. In addition to the above, sleep helps us overcome traumatic memories, reduces athletes' injury rates, makes us look more attractive, reduces food cravings, and so on and so on...

The evolution of sleep

  • I guess it shouldn't be a surprise that sleep is so broadly useful: once it started, it makes sense that many metabolic processes would take advantage of it. And they've had a long time to do so: sleep is ancient, with all animal species demonstrating some form of sleep-like behaviour. 
  • Even unicellular bacteria have active and passive phases corresponding to the planet's light/dark cycle.
  • However, the length of sleep required varies wildly for different animals, from 4 hours for elephants to 19 for brown bats. 
  • Only birds and mammals have proper REM sleep - it is a relatively recent adaptation. It also seems to be absent in aquatic mammals, whose two brain hemispheres sleep separately. 
  • Humans seem to be naturally biphasic: modern hunter-gatherer tribes sleep for 7-8 hours at night, and then nap for 30-60 minutes in the afternoon. It's biologically natural to be sleepy after lunch. Biphasic sleep significantly decreases mortality from heart disease. 
  • Walker hypothesises that descending from the trees to sleep on the ground allowed us to gain more REM sleep (particularly difficult in trees due to sleep paralysis), and therefore was important in boosting human cognitive development; also, that fire was vital in making ground-sleeping safer.

How to sleep better

  • Alcohol is an extremely powerful suppressor of REM sleep. Since it stays in your system for hours, it's best not to drink in the evenings. 
  • Light, especially blue light, signals your circadian system to wake up. Unfortunately LED screens provide a lot of blue light. Avoid using screens in the hours before bed, or at least phase out the blue light (e.g. using flux). 
  • In addition to light, our bodies use decreasing temperatures as a signal to sleep. Lowering room temperature often helps with insomnia. Apparently your core temperature will also fall after a hot bath. 
  • Caffeine has a half-life of 5 to 7 hours, so if you drink it in the afternoon, a significant amount will still be in your system at bedtime. 
  • The circadian rhythm of a teenager is naturally a few hours later than that of an adult, so teens shouldn't be forced to get up too early. Unfortunately, schools aren't taking much notice of this. 
  • Apparently sleeping pills cause lower-quality sleep and have severe long-term side-effects, so they should be avoided (with the exception of melatonin). 
  • For serious sleep problems, Cognitive Behavioural Therapy for Insomnia (CBT-I) works fairly well and should be the first step.

As you can probably tell from the above, Walker is very much a cheerleader for sleep. This does bias him in some noticeable ways - e.g. his overt scorn towards coffee. He also blurs causation and correlation at some points throughout the book, so I'd be surprised if all of the deleterious effects mentioned above are as significant as he claims. But the overall picture is stark enough that I'm now very worried about the ongoing sleep loss epidemic.

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18 comments, sorted by Click to highlight new comments since: Today at 10:08 AM

I think I would love to see an epistemic spot-check in the style of AcesoUnderGlass for this book. If anyone is up for it, I am happy to put out a $50 bounty for anyone writing up such a post and post it to LessWrong.(Independent of quality, as long as it's not completely egregious. If more than one person ends up writing one, then I am happy to pay out the price up to three times)

Note that guzey's excellent writeup on this definitely qualifies, and I offered to send him the money, but if I remember correctly he didn't want it, and we will settle it informally when we hang out in the future sometime.

Note that the book contains a multitude of basic scientific errors, misrepresentation of research, data manipulation, etc: https://guzey.com/books/why-we-sleep/

The most in-depth, but a bit outdated (c. 2012) article on sleep is written by Piotr Wozniak, whom you might know as a pioneer of spaced repetition software. The article is ~300 pages long. It includes summary & myths sections which are a bit longer than this post.


Just a side question here about the "a bit outdated (c. 2012)" note. Is that because you think the science/level of knowledge or some other technology related to such studies is changing that quickly?

Both the reference to additional sources and the review were great. Thanks to both you and ricraz.

You are welcome! A general concern about the pace of scientific progress.

Wikipedia page for 'Cognitive behavioral therapy for insomnia' is a great source of useful sleep related habits.

Note that this book is apparently riddled with errors.


Good book. Would recommend.

My major take away is that sleep problems are usually psychological not Physiological. Usually people don't know how to stop a busy mind and sleep. This leads to endless rumination and being held awake by stress. We don't teach the skill and most people just figure it out on their own.

Also extra sleep tips here: http://bearlamp.com.au/a-very-long-list-of-sleep-maintenance-suggestions/

Thanks for the interesting review summary. Does he say a bit more about the effects of napping / biphasic sleep vs monophasic sleep?

I'll check when I have a copy at hand. From memory, he says that the "biphasic" sleep where you wake up in the middle of the night for an hour or so has no scientific backing. I think his main evidence for the health benefits of of afternoon naps is looking at the rates of heart disesase in Mediterranean countries as their sleep patterns shift. This doesn't seem to be particularly rigorous, but I recall the effect size being pretty large.

Great post! I generally find book reviews and book summaries quite useful, and am happy to see more of them. On the object level, I am vaguely remembering a study that I can't find right now, that added something interesting to the sleep question, which was something like this:

"We had three test-groups, one of which slept normally at about 8 hours a night, one of which slept for 7 hours a night, and one of which slept for 6 hours a night, for a week. The 7 hour group started out with a similar performance to the 8 hour group, but went down to the performance of the 6 hour group after about 4 days. However, the self-assessment of how sleep-deprived the individuals were was quite accurate for the 6 hour group, but didn't identify any worsening aspects of sleep deprivation for the 7 hour group, even after 4 days of testing. This suggests that subjects are quite bad a assessing mild sleep-deprivation, even after prolonged exposure."

I wonder whether the book made any reference to that study, since I've been looking for a while, and if anyone can find it, than I do remember it being a significant update on how much I trust myself to assess how much sleep I need.

In chapter 7 (Too Extreme for the Guinness Book of World Records) Walker mentions a research by David Dinges which seems kind of similar to what you described. I didn't find a reference in the book, but I found this highly cited paper, which seems to me like the one he was referring:

The Cumulative Cost of Additional Wakefulness: Dose-Response Effects on Neurobehavioral Functions and Sleep Physiology From Chronic Sleep Restriction and Total Sleep Deprivation


Walker mentions a research by Gregory Belenky with almost identical results that was published around the same time. I found a highly cited paper which seems to me like the one he was referring:

Patterns of performance degradation and restoration during sleep restriction and subsequent recovery: a sleep dose‐response study



Walker has a sub-title "you do not know how sleep-deprived you are when you are sleep-deprived" in chapter 7, so you can guess what the researches above found.

Excellent, these really do like the studies that I remember reading. Thank you a lot!

I would be glad to send you $10 via PayPal if you want, since I've been looking for these for quite a while.

No thanks. Knowing I helped you out is exactly the right reward for me :)

Thanks for the review!

I am only halfway through the book, but I am already firmly resolved to highly prioritize 8 hours of sleep each night.

I would encourage everyone to read the book while experimenting with getting enough sleep consistently (e.g., for a week), and see for yourself whether the short-term benefits are worth it.

Maybe my assessment is not accurate, but it seems to me that "8 hours of sleep each night is extremely beneficial (relative to other stuff you could do)" is not a mainstream belief among Less Wrong readers. (Though I could find examples, e.g., http://www.thebayesianconspiracy.com/2018/06/61-biohacking-101/ and https://www.lesswrong.com/posts/KsrTZtu48qdkAeSht/a-few-tips-on-how-to-generally-feel-better-and-avoid .)

I would love to hear that I am completely wrong here, but if I am not, then why is that? Isn't the evidence for the benefits of sleep as overwhelming as the book describes it?

Even after correcting for the book's bias toward favoring sleep (note that I am not qualified to assess the existence/size of such bias), the evidence still seems overwhelming to me.

A few things that I would add to your great summary:


The book gives another argument for the huge benefits of sleep, which I find quite intuitive:

Addressing the question of why we sleep from an evolutionary perspective only compounds the mystery. No matter what vantage point you take, sleep would appear to be the most foolish of biological phenomena. When you are asleep, you cannot gather food. You cannot socialize. You cannot find a mate and reproduce. You cannot nurture or protect your offspring. Worse still, sleep leaves you vulnerable to predation. Sleep is surely one of the most puzzling of all human behaviors.
On any one of these grounds—never mind all of them in combination—there ought to have been a strong evolutionary pressure to prevent the emergence of sleep or anything remotely like it. As one sleep scientist has said, “If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made.”
Yet sleep has persisted. Heroically so. Indeed, every species studied to date sleeps.


I think this quote from the book is also of particular interest to Less Wrong readers:

You Do Not Know How Sleep-Deprived You Are When You Are Sleep-Deprived
The third key finding, common to both of these studies, is the one I personally think is the most harmful of all. When participants were asked about their subjective sense of how impaired they were, they consistently underestimated their degree of performance disability.

(As mentioned in another comment, I think the studies that Walker refers to here are these two:

The Cumulative Cost of Additional Wakefulness: Dose-Response Effects on Neurobehavioral Functions and Sleep Physiology From Chronic Sleep Restriction and Total Sleep Deprivation ( https://academic.oup.com/sleep/article/26/2/117/2709164 )

Patterns of performance degradation and restoration during sleep restriction and subsequent recovery: a sleep dose‐response study ( https://onlinelibrary.wiley.com/doi/full/10.1046/j.1365-2869.2003.00337.x )


I would summarize the following quote by "You might believe that your current state is close to your optimal state, but you just forgot (or never knew) how much better life could be if you got enough sleep consistently":

Similarly problematic is baseline resetting. With chronic sleep restriction over months or years, an individual will actually acclimate to their impaired performance, lower alertness, and reduced energy levels. That low-level exhaustion becomes their accepted norm, or baseline. Individuals fail to recognize how their perennial state of sleep deficiency has come to compromise their mental aptitude and physical vitality, including the slow accumulation of ill health. A link between the former and latter is rarely made in their mind. Based on epidemiological studies of average sleep time, millions of individuals unwittingly spend years of their life in a sub-optimal state of psychological and physiological functioning, never maximizing their potential of mind or body due to their blind persistence in sleeping too little.
Humans seem to be naturally biphasic: modern hunter-gatherer tribes sleep for 7-8 hours at night, and then nap for 30-60 minutes in the afternoon.

I'm having fun imagining a workplace where this sort of pattern is encouraged. A large part of the difficulty would be dealing with self-destructive work cultures where nobody wants to violate a norm in a way that risks making them look lazy. I'd want to start by having the highest performers try napping after lunch, so that people come to associate it positively with productivity, frame napping as the opposite of lazy, like exercise, it's not work but it's something that hard workers do. But there's a chance the higher-performers are exactly the people who wouldn't benefit from napping- There is such a thing as a short-sleeping gene in humans and a lot of CEOs seem to have it- so starting with them might soil the whole thing.

I'm remembering hearing stories of a lot of workplaces getting nap pods and telling their employees that it is "okay" to nap. I don't think this should be taken seriously. If you don't have enough beds for everyone in the office to sleep, it wont become a norm. It certainly wont become a habit.

I'd want to experiment with assigning a sample of people (or a set of volunteers) to napping every day, that's a design we could take seriously.

Thank you for the write up! Matthew Walker was also on The JRE Podcast and has a Talk at Google. Well worth checking out.