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Why We Sleep by Matthew Walker Summary

Sleep enhances our ability to learn, memorize, and make logical decisions. It recalibrates our emotions, restocks our immune system, fine-tunes our metabolism, and regulates our appetite. Lack of sleep is associated with higher rates of depression, anxiety, immune system failure, stroke, heart failure, cancer, dementia, skin problems, and overeating.


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1-Page Book Summary of Why We Sleep

Sleep is universal in animals (even in insects and worms) — this alone suggests a vital function and that it isn’t simply a vestigial byproduct of evolution.

In the current nutrient-rich environment, humans, in general, need 8 hours of sleep to function optimally.

  • True low-sleepers (chronically < 6 hours of sleep/night without impairment of function) are incredibly rare, less than 1% of the population. Everyone else is disguising their sleep deprivation with caffeine and sleeping pills.
  • Insidiously, you’re very bad at objectively assessing your decrease in performance under sleep deprivation.
  • Fasting and starvation does lower sleep, which is why hunter-gatherer tribes show 6.5 hours of sleep (which is then picked up by popular media)

Sleep has two general types: Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM)

  • NREM occurs earlier in the sleep phase, while REM is concentrated later.
  • NREM is slow (~2Hz) (like billions of neurons singing in synchrony) while REM is fast (50Hz) and looks like being awake.
  • NREM is responsible for pruning memories, transferring short-term memory to long-term memory, gaining “muscle memory,” growth hormone secretion, and parasympathetic nervous system activation.
  • REM is responsible for forming new neural connections, problem-solving, dreaming, blunting emotional responses to painful memories, reading other people’s facial emotions, and neonatal synaptogenesis.
  • Both are generally necessary — depriving a person of either one leads to different problems.
Summary of Why We Sleep

Sleep deprivation shows consistently bad outcomes. Nothing is reported to be beneficial from sleep deprivation.

  • higher mortality, risk of cancer, heart disease, weight gain, rate of infection, Alzheimer’s, irritability, inflammation.
  • lower productivity, social fluidity, rational decisionmaking, memory recall, emotional control, testosterone, immune system function, response to the flu vaccine.
  • in the extreme, chronic sleep deprivation causes death.
  • even the apparent increase in wakeful time to be productive is balanced by lower productivity and creativity.

How to improve your sleep

  • Sleep and wake up at the same times every day.
  • Reduce light before sleep. Blue light is the most harmful, but even bedside lamps cause issues. Artificial light delays the circadian rhythm by hours.
  • Cool body temperature before sleep. The ideal sleeping temperature is 65°F (19°C) given standard bedding and clothes. Other tricks: expose your palms and feet while sleeping, take a hot bath before sleeping.
  • Don’t drink alcohol unless it is completely metabolized by sleep time (including the aldehydes produced).
  • Don’t drink caffeine at all, but especially not in the afternoon.
  • Don’t use alarms if you can help it. Alarms cause a huge stress reaction on waking. Snoozing causes repeated traumas every morning.
  • Exercise regularly, but not 3 hours before sleep.
  • Don’t rely on sleeping pills — these are usually just sedatives that put you more in a sedation state than sleep.

Interesting future sleep-related ideas:

  • In cars, detect sleep-deprived driving and react like drunk driving (shut it down, increase insurance premiums)
  • An automated household that adjusts light and temperature to each person’s circadian rhythm
  • If one can induce sleep naturally, can relieve not just insomnia but also PTSD and substance abuse

What Happens to Your Brain When You Dream

What is dreaming and what happens and are there any real benefits to dreaming? Well, to take a step back I think it’s important to note that dreaming essentially is a time when we all become flagrantly psychotic. And before you perhaps dismiss that diagnosis, I’ll give you five good reasons, because last night when you were dreaming, first you started to see things which were not there, so you were hallucinating.

Second, you believe things that couldn’t possibly be true, so you were delusional. Third, you became confused about time, place, and person, so you’re suffering from disorientation. Fourth, you had wildly fluctuating emotions like a pendulum, something that we call being affectively labile. And then, how wonderful? You woke up this morning and you forgot most if not all of that dream experience, so you’re suffering from amnesia.

And if you were to experience any one of those five symptoms while you were awake, you would be seeking psychological or psychiatric treatment, yet during sleep and dreaming it seems to be both a normal biological and psychological process.

What are the functions, then, or the benefits of dreaming? Well we know that dream sleep, which principally comes from a stage that we call rapid eye movement sleep or REM sleep, dream sleep actually provides at least two benefits for the brain.

The first is actually creativity because it’s during REM sleep and dreaming specifically when the brain starts to collide all of the information that you’ve recently learned together with all of this back catalog of autobiographical information that you’ve got stored up in the brain. And it starts to build novel connections, it’s almost like group therapy for memories. And through this pattern of informational alchemy at night, we create a revised mind wide web of associations. And you can start to divine new novel insights into previously unsolved problems so that you wake up the next morning with new solutions, and it’s probably the reason that no one has ever told you that you should stay awake on a problem. Instead, people tell you to sleep on a problem. And we now have good evidence that it’s dream sleep that gifts you that type of informational wisdom rather than simply knowledge.

“Sleep is the single most effective thing we can do to reset our brain and body health each day — Mother Nature’s best effort yet at contra-death.” 

The second benefit of dream sleep is essentially a form of overnight therapy. It’s during dream sleep where we start to actually take the sting out of difficult, even traumatic, emotional experiences that we’ve been having. And sleep almost divorces that emotional, bitter rind from the memorable experiences that we’ve had during the day. And so that we wake up the next morning feeling better about those experiences. So you can think of dream sleep as emotional first aid and it sort of offers this nocturnal soothing balm that smoothes those painful stinging edges of difficult experiences. So it’s not time that heals all wounds, but it’s time during dream sleep that provides you with emotional convalescence.

5 Tips for Falling Asleep Quicker

What are things that we can all do tonight and in the future to start getting better sleep? Well, beyond carving out a non-negotiable eight-hour sleep opportunity, there are probably at least five things that we can do.

The first is that we have to try and maintain regularity. And if there’s one thing that you take away from this, it would be going to bed at the same time and waking up at the same time, no matter whether it’s the weekday or the weekend. Even if you’ve had a bad night of sleep, still wake up at the same time of day and reset.

The second thing is that we are a dark-deprived society in this modern era and we need darkness in the evening to allow the release of a hormone called melatonin. And melatonin helps the healthy timing of our sleep.

So try to dim down half the lights in your home in the hour before bed. Stay away from screens, especially those LED screens — they emit blue light that actually puts the breaks on melatonin. And those blue-light emitting devices fool your brain into thinking that it’s still daytime, even though it’s nighttime and you want to get to sleep.

“Humans are not sleeping the way nature intended. The number of sleep bouts, the duration of sleep, and when sleep occurs has all been comprehensively distorted by modernity.” 

The third key ingredient is to keep it cool. Many of us actually have a bedroom that’s too warm in terms of temperature.

So an optimal temperature is about 68 degrees Fahrenheit or about 18 and a half degrees Celsius. And the reason is that your brain and your body need to drop their core temperature by about two or three degrees Fahrenheit to initiate good sleep. And that’s the reason why you’ll always find it easier to fall asleep in a room that’s too cold than too hot. So having a cool room actually takes your brain and body in the right temperature direction to get good sleep.

The fourth critical factor is actually avoiding alcohol and caffeine. Unfortunately, this makes me deeply unpopular but alcohol is perhaps the most misunderstood drug when it comes to sleep. People think that it helps them fall asleep. That’s not actually true. Alcohol is a class of drugs that we call, “the sedatives.” And what you’re doing is just knocking your brain out. You’re not putting it into natural sleep.

We also know that alcohol will fragment your sleep. So you’ll wake up many more times throughout the night. And alcohol is also a very potent chemical for blocking your dream sleep or your rapid eye movement sleep.

Caffeine is also a problem. Many of us know that caffeine can keep us awake. It’s an alerting chemical, it’s a stimulant in terms of a class of drugs. But few people know that even if you can have a cup of coffee after dinner and you fall asleep fine and maybe you stay asleep, the depth of the deep sleep that you have when there is caffeine within your brain isn’t as deep as when you’ve abstained from that cup of coffee after dinner. So as a consequence, you wake up the next morning, you feel unrefreshed and you don’t remember waking up or having a difficult time falling asleep but now you find yourself reaching for two or three cups of coffee in the morning and you develop this dependency, this addiction cycle.

The fifth and final tip for better sleep is to not stay in bed awake. So if you haven’t fallen asleep within 20 or so minutes or you’ve woken up and you’re finding it difficult to fall back asleep, don’t stay in bed awake. The reason is that your brain very quickly starts to learn the association between your bed being about the place that you’re awake rather than your bed being about sleep. So the advice is to get up, go to another room and in dim light, just read a book. No screens, no email checking, no food. And only when you feel sleepy should you return to bed and that way you can then actually re-learn the association between your bedroom being about the place of sleep rather than being awake.

I should also note that some people don’t like the idea of getting up and going out to a different room if it’s dark and they’re warm in bed.

An alternative is actually meditation. Meditation has been demonstrated in clinical trials to help people just relax the body, calm down the fight-or-flight branch of the nervous system that can happen when we wake up in the middle of the night and we have that Rolodex of anxious thoughts. And by meditating, you can start to quiet the mind as well as the body and that also helps you fall back asleep more easily.

The Dangers of Sleep Deprivation

We certainly know that a lack of sleep will actually prevent your brain from being able to initially make new memories, so it’s almost as though without sleep the memory inbox of the brain shuts down and you can’t commit new experiences to memory. So those new incoming informational emails are just bounced, and you end up feeling as though you’re amnesiac. You can’t essentially make and create those new memories.

We also know that a lack of sleep will lead to an increased development of a toxic protein in the brain that is called beta-amyloid and that is associated with Alzheimer’s disease because it is during deep sleep at night when a sewage system within the brain actually kicks in to high gear and it starts to wash away this toxic protein, beta-amyloid.

So if you’re not getting enough sleep each and every night, more of that Alzheimer’s-related protein will build up. The more protein that builds up, the greater your risk of going on to develop dementia in later life.

What are the effects of sleep deprivation on the body? Well, there are many different effects. Firstly, we know that sleep deprivation affects the reproductive system. We know that men who are sleeping just five to six hours a night have a level of testosterone which is that of someone ten years their senior. So a lack of sleep will age you by almost a decade in terms of that aspect of virility and wellness.

We also know that a lack of sleep impacts your immune system. So after just one night of four to five hours of sleep, there is a 70% reduction in critical anticancer-fighting immune cells called natural killer cells. And that’s the reason that we know that short sleep duration predicts your risk for developing numerous forms of cancer. And that list currently includes cancer of the bowel, cancer of the prostate, as well as cancer of the breast.

“the shorter your sleep, the shorter your life. The leading causes of disease and death in developed nations—diseases that are crippling health-care systems, such as heart disease, obesity, dementia, diabetes, and cancer—all have recognized causal links to a lack of sleep.” 

In fact, the link between the lack of sleep and cancer is now so strong that recently the World Health Organization decided to classify any form of nighttime shift work as a probable carcinogen. So in other words, jobs that may induce cancer because of a disruption of your sleep rate rhythms.

We also know that a lack of sleep impacts your cardiovascular system because it is during deep sleep at night that you receive this most wonderful form of effectively blood pressure medication. Your heart rate drops, your blood pressure goes down.

If you’re not getting sufficient sleep, you’re not getting that reboot of the cardiovascular system, so your blood pressure rises. You have if you’re getting six hours of sleep or less, a 200% increased risk of having a fatal heart attack or stroke in your lifetime.

There is a global experiment that is performed on 1.6 billion people twice a year and it’s called daylight savings time. And we know that in the spring when we lose one hour of sleep we see a subsequent 24% increase in heart attacks the following day.

Another question, perhaps, is what is the recycling rate of a human being? How long can we actually last without sleep before we start to see declines in your brain function or even impairments within your body? And the answer seems to be about 16 hours of wakefulness.

Once you get past 16 hours of being awake, that’s when we start to see mental deterioration and physiological deterioration in the body. We know that after you’ve been awake for 19 or 20 hours, your mental capacity is so impaired that you would be as deficient as someone who was legally drunk behind the wheel of a car. So if you were to ask me what is the recycle rate of a human being, it does seem to be about 16 hours and we need about eight hours of sleep to repair the damage of wakefulness. Wakefulness essentially is low-level brain damage.

Melatonin, Jet Lag And The Two Systems That Trigger Sleep

If you’ve ever had any kind of sleep problem, you’ve probably browsed the supplements section at your local pharmacy. Most common among those supplements? Melatonin.

But does melatonin actually work?

The author recommends taking melatonin only if you’re going to be traveling and experiencing jet lag. To understand why you need to know how the body actually goes to sleep in the first place.

You’ve probably heard the term “circadian rhythm” before. Basically, your circadian rhythm is your level of energy or wakefulness throughout the day. Circadian rhythms are kind of a cycle: most people will feel awake in the morning, experience a dip in the early afternoon, feel awake in the early evening and then get tired around bedtime.

Recent research has increased our understanding of the circadian rhythm by quite a lot. Actually, the 2017 Nobel Prize in Physiology or Medicine was awarded to circadian rhythm researchers. The whole system is pretty complicated, but at a simple level, our bodies produce melatonin to signal when it’s time for us to sleep.

Summary of Why We Sleep
Summary of Why We Sleep

In general, we’re better off trying to stimulate natural melatonin production than taking outside melatonin. Part of the reason is that there’s a second system our body uses for sleep.

When we’re awake, our bodies gradually increase the concentration of adenosine in our brains. As more adenosine builds up, we feel more “sleep pressure,” or desire to sleep. The longer you stay awake, the more sleep pressure you experience, which is why you can still fall asleep in the middle of the day if you’re truly exhausted.

When you sleep, your body dumps adenosine. In an ideal situation, you dump adenosine during a quiet period of your circadian rhythm, so that you can have a truly deep sleep.

Summary of Why We Sleep
Summary of Why We Sleep

Why does jet lag feel worse flying eastbound?

This is because you need to fall asleep when the body wants to be awake which is a more difficult task than staying awake when the body wants to sleep.

Also, because the circadian rhythm is slightly longer than a day, lengthening it is easier than shortening it.

Morning Larks or Night Owls

Everyone has a different circadian rhythm. For some people, their peak of wakefulness arrives early in the day, and their sleepiness trough arrives early at night. These are “morning types,” and make up about 40% of the population. These people tend to wake around dawn and are happy to do so and function optimally at this time of day.

Other people are “evening types” and account for about 30% of the population. These people prefer going to bed late and wake up late the following morning.

The remaining 30% of the population is somewhere between a morning and evening type.

Your “type,” i.e. whether you’re a morning or evening person, is known as your chronotype and it’s largely determined by genetics. If you’re a night owl, it’s likely that one or both of your parents is a night owl.

Sadly, society treats night owls as lazy since they don’t like to wake up until later in the morning. In addition, society’s work schedule favors morning types. Evening types are often forced into an unnatural sleep-wake rhythm to meet a certain work schedule, thus evening types are more often sleep-deprived.

You may be wondering why Mother Nature would program this variability across people. As a social species, should we not all be synchronized and therefore awake at the same time to promote maximal human interactions? Perhaps not. Humans likely evolved to co-sleep as families or even whole tribes, not alone or as couples. The benefits of such genetically programmed variation in sleep/wake timing preferences can be understood. The night owls in the group would not be doing to sleep until one or two a.m., and not waking until nine or ten a.m. The morning larks, on the other hand, would have retired for the night at nine p.m. and woken at five a.m. Consequently, the group as a whole is only collectively vulnerable (i.e., every person asleep) for just four rather than eight hours, despite everyone still getting the chance for eight hours of sleep. That’s potentially a 50 percent increase in survival fitness.

“Asking your teenage son or daughter to go to bed and fall asleep at ten p.m. is the circadian equivalent of asking you, their parent, to go to sleep at seven or eight p.m. No matter how loud you enunciate the order, no matter how much that teenager truly wishes to obey your instruction, and no matter what amount of willed effort is applied by either of the two parties, the circadian rhythm of a teenager will not be miraculously coaxed into a change. Furthermore, asking that same teenager to wake up at seven the next morning and function with intellect, grace, and good mood is the equivalent of asking you, their parent, to do the same at four or five a.m.” 

Lucid Dreaming

By definition, lucid dreaming is simply the act of knowing that you’re dreaming whilst you’re dreaming.

Most people actually think of lucid dreaming more in the sense of actually beginning to control what you’re dreaming. So, you gain volitional control and you decide what’s going to happen during your dream.

Scientists have designed experiments and they’ve been able to demonstrate objectively that when people say that they’re doing something in that dream that they actually are.

How can we actually become more capable of lucid dreaming? Well, it’s a little bit tricky, but you can certainly try to tell yourself that you will remember that you’re dreaming whilst you’re dreaming before you actually fall asleep. So, try to go through a mantra chant as it were. Some people actually try to do deliberative things whilst sleeping, like turning on the lights in a room. And that helps them to become aware that they are dreaming at the moment of dreaming itself and therefore they gain lucid control.

It seems to be only around 20 to 30% of the population are actually natural lucid dreamers. So, perhaps if it was so beneficial mother nature would have had all of us being able of lucid dreaming. And the fact that we’re not perhaps means that it’s not necessarily beneficial. But we actually don’t know. Maybe those 20 to 30% of people who do lucid dream are at the forefront of hominid evolution, and they are going to be the next species of preference. We just don’t know.

Why Naps Can Be Dangerous for Your Health

Should we actually be taking naps during the day? Well, we certainly know from evidence in my own sleep center and that of many other scientists that naps can give you benefits for both your brain and your body. But naps can actually be a double-edged sword because whilst we’re awake during the day, we’re building up sleepiness or sleep pressure. So that when you try to fall asleep at night, you’ll fall asleep quickly and then you’ll stay asleep. And when we sleep, we actually release that sleepiness, almost like a valve on a pressure cooker, so that we wake up the next morning feeling refreshed.

So if you take a nap during the day, especially if you take it too late in the afternoon, you will actually release some of that sleepiness and it will make it that much more difficult to fall asleep and stay asleep soundly throughout the night. So the advice would be if you don’t struggle with your sleep and you can nap regularly, then naps are just fine.

But if you do find it difficult to fall asleep or stay asleep at night, then you should avoid naps and try and build up that healthy sleepiness in the evening.

The ideal nap depends on what you want from that nap. Different stages of sleep actually give you different types of brain and body benefits.

But if you want to avoid that grogginess that you can often have after a long sleep, then you should perhaps avoid naps that are longer than maybe 40 or 50 minutes in length.

You should also try to avoid naps late in the afternoon so that you wake up at least after you finish the nap and still have enough time to build up that sleepiness, that sleep pressure so you can get to sleep in the evening.

“They discovered that naps as short as twenty-six minutes in length still offered a 34 percent improvement in task performance and more than a 50 percent increase in overall alertness.” 

Can naps help with sleep debt?

And the answer, unfortunately, is no. Sleep is actually not like the bank. You can’t accumulate a debt and then hope to pay it off at some later point in time. So sleep is an all-or-nothing event in that sense. So you can’t short sleep during the week and then try to binge and oversleep at the weekend.

So if I were to take a human being and deprive them of sleep for one night, so that they’ve lost eight hours of sleep and then I give them all of the recovery sleep that they want on a second, a third, even a fourth night, they do sleep longer but they never get back that full eight hours that they’ve lost.

And we can ask then, “Why isn’t there some kind of a credit system in the brain? Why don’t we have a cell that can actually store up that sleep?”

Because there is a precedent for this. We actually have a storage system for calories within the body and they’re called, “fat cells.” Because there was a time during the evolutionary process where we had feast and where we had famine. And when we had feast, we would store that caloric energy and fat cells that when we went into debt during that caloric famine, we could actually survive.

Human beings are the only species that deliberately deprive themselves of sleep for no apparent reason. And that means that Mother Nature has never had to face the challenge of sleep deprivation during the course of evolution and therefore, she’s never had to come up with a safety net mechanism that overcomes a sleep debt.


If you’re having trouble or things feel overwhelming, check your sleep. It’s a subtle cause that may have flown under the radar. And if you want to learn more about how sleep affects you, check out the full book.


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Why We Sleep Summary
Summary of Why We Sleep

Adapted from:
Detailed chapter-by-chapter summary (47–60 min estimated reading time) by Allen Cheng

A sleep expert explains what happens to your brain when you dream

A sleep expert explains what happens to your body and brain if you don’t get enough sleep

How to know if you’re sleep deprived, even if it doesn’t feel like it

A sleep expert gives the best tips for falling asleep quicker

How you can control your dreams – an expert explains how lucid dreaming works

A sleep expert explains why naps can be dangerous for your health

Why We Sleep Book Review and Summary: Why Sleep Might Be Your Life’s Missing Ingredient – Routine…

Why We Sleep by Matthew Walker: Summary and Notes – Four Pillar Freedom

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