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Why We Sleep by Matthew Walker.
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1-Page Summary 1-Page Book Summary of Why We Sleep

Sleep is universal in animals (even in insects and worms). These deep biological roots suggest a vital function and that it isn’t simply a vestigial byproduct of evolution.

Humans in today’s nutrient-rich environment need 8 hours of sleep to function optimally.

  • True low-sleepers (people who can chronically sleep < 6 hours per night, without impairment of function) are incredibly rare, at 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. When sleep-deprived, you think you’re performing better than you really are.
  • Fasting and starvation do lower sleep, which is why hunter-gatherer tribes show 6.5 hours of sleep. This fact is then inappropriately picked up by popular media as evidence that it’s natural. But when resources are plenty, your body naturally wants to sleep more.

Sleep has two general types - NREM and REM.

  • 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.

**Sleep deprivation shows consistently bad...

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Why We Sleep Summary Chapter 2: Your Daily Sleep Rhythm

(Chapter 1 is just an introduction, so we’re skipping it.)

Sleep is universal in animals, even in insects and worms, despite its apparent drawbacks (vulnerability to predators, loss of time for productivity). When a biological feature is preserved deep in evolutionary history, it is usually a critical function. This must mean sleep is a critical function, and it’s important to understand why it’s important.

How Sleep Rhythm Works

Sleep is regulated by two mechanisms:

  • The circadian rhythm, regulated by melatonin (produced by the suprachiasmatic nucleus in the brain). Think of this as a natural “wake drive,” making you stay awake during the day and waning during night.
    • The circadian rhythm responds to light and darkness to calibrate itself. It’s naturally 24 hours and 15 minutes long on average.
  • Adenosine is a fatigue signal and causes “sleep pressure.” This rises consistently through the day without sleep, making you feel more tired. Sleeping depletes adenosine, and you wake up with a lower level.

These two mechanisms interact as shown in this...

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Why We Sleep Summary Chapter 3: Sleep Cycles within a Night

Now you understand how your sleep rhythm gives a regular schedule of sleep from night to night. Next, we’ll look into how, within a single night of sleep, your brain cycles between different types of phases of sleep. This is important to understanding the function of sleep for your brain.

In summary, your brain switches between two types of sleep - REM (rapid eye movement) and non-REM (NREM) sleep. The two types of sleep have different functions - simplistically:

  • NREM clears out old memories and mental “trash,” and it moves information into long-term storage.
  • REM strengthens the valuable connections that remain, and it forges creative novel connections.

In total for a single night, there’s about an 80/20 split between NREM/REM sleep.

When you sleep, your brain goes through sleep cycles that each last about 90 minutes. Each sleep cycle generally begins with NREM sleep, then ends with REM sleep. As one cycle ends, the next begins. You can see this in a sleep graph here:


A deeper line means going deeper into NREM sleep.

How Sleep Cycles Change Through the...

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Why We Sleep Summary Chapter 4: Interesting Facts About Sleep

Sleep in Animals

Sleep is present in all animal species, even invertebrates. And bacteria that survive for longer than 24 hours have circadian-like rhythms. As we’ve said before, this suggests that it’s universally critical for survival.

We ask all the time why we sleep. One researcher posed an interesting inversion of the question - if wakefulness is damaging to the body and sleep recovers it, why did life ever bother to wake up? (Shortform note: of course, you can’t be productive and reproduce when sleeping, so sleeping too much would be evolutionarily disadvantageous.)

The amount of sleep per day varies from 4 hours in elephants to 19 hours in bats. There are no strong correlations between animal characteristics and amount of sleep, though within animals of a similar size, a more complex brain increases sleep.

Among animals, REM sleep occurs only in birds and mammals. Because REM evolved independently in these two distant evolutionary trees, REM likely performs a critical function that NREM cannot accomplish, or that REM is more efficient at accomplishing.

Interesting animal sleep patterns

  • Cetaceans (dolphins, whales) sleep with half their...

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Why We Sleep Summary Chapter 5: How Sleep Changes from Childhood to Adulthood


Fetuses spend almost all of the time in a sleep-like state. It doesn’t yet have the part of the brain that causes muscle-atonia during sleep, thus explaining why babies in the womb kick and punch.

During the last 2 weeks of pregnancy, REM sleep in fetuses ramps up to 12 hours a day. This causes rapid synaptogenesis and building of neural pathways throughout the brain. In experiments with rat fetuses, disturbing REM sleep stalls construction of the cerebral cortex.

Alcohol impedes REM sleep in fetuses and babies, causing abnormal synaptogenesis. Once disrupted, a fetal brain may never fully regain normal function.

  • Newborns of alcoholic mothers spend far less time in REM sleep.
  • 2 drinks reduce REM sleep and breathing rate in unborn infants.
  • When babies drink milk containing alcohol, their REM sleep reduces by 30%.

Because REM sleep is involved in emotional recognition and social interaction, disrupting REM sleep in utero might contribute to the autism spectrum.

  • Autistic people show 30-50% less REM sleep than normal.
  • Rats deprived of REM sleep develop into socially withdrawn adults.



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Why We Sleep Summary Chapter 6: How Sleep Benefits the Brain

Getting good sleep improves your brain in these ways:

1) Sleep improves long-term factual recall

Your brain stories different memories in different places. The hippocampus stores short-term memory with a limited capacity; the cortex stores long-term memory in a large storage bank.

The slow-wave, pulsating NREM sleep moves facts from the hippocampus to the cortex. This has two positive effects: 1) it secure memories for the long term, and 2) it clears out short-term memory to make room for new information and improves future learning.

Have you ever woken up recalling facts that you couldn’t have recalled before sleeping? Sleep may make corrupted memories accessible again.

While good sleep improves memory, sleep deprivation can prevent new memories from being formed. In part, this might be because the hippocampus becomes less functional with less sleep, partially because lack of NREM sleep prevents solidifying of new memories.

Unfortunately, making up a sleep deficit later doesn’t help recover a previous days’...

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Why We Sleep Summary Chapter 7: How Sleep Deprivation Harms the Brain

While getting great sleep is good for the brain, sleep deprivation is unambiguously harmful to the brain. We’ll show damage in three ways: to attention, to emotion control, and for Alzheimer’s Disease.

Sleep Deprivation Worsens Attention and Concentration

Sleep deficits are very bad for attention and concentration. This is especially harmful during high-risk activities, like driving.

Here are some ways to put the risk into perspective:

  • Driving after having slept less than 4 hours increases crash risk by 11.5x.
  • Being awake for 19 hours (being past your bedtime by 3 hours) is as cognitively impairing as being legally drunk.
  • Adding alcohol to sleep deficits has a multiplicative effect on mistakes, not just an additive one.

Sleep deficits add up over time, and performance progressively worsens with greater sleep deficit. Having 10 nights of 6-hour sleep is equal in damage to one all-nighter, as is 6 nights of 4-hour sleep.

Why does sleep cause more accidents? Part of it is delayed reaction time. Another part is a “microsleep,” where your eyelids shut for just a few seconds and you go unconscious and lose motor control. If you’re in...

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Why We Sleep Summary Chapter 8: How Sleep Deprivation Harms the Body

In addition to the damage it causes to the brain, sleep deprivation disrupts the normal function of many physiological processes, likely contributing to chronic diseases. In this chapter, we’ll cover a bevy of health issues associated with sleep deprivation.

At a high level, sleep deprivation of even just 1-2 hours triggers the sympathetic nervous system (fight or flight response) and disrupts hormonal balances. This also implies that sleep is necessary for the normal maintenance of physiology.

Shortform Caveat

Many of the population studies cited in Why We Sleep are correlational - e.g. people who sleep less are more likely to have heart disease, after controlling for many other factors. But the causation is unclear - some other factors that predispose people to get heart disease (like a high baseline level of stress) could also reduce sleep.

To address this, the experimental studies cited attempt to link lack of sleep to a middle physiological state, which itself is causative for the disease. For instance, lack of sleep increases blood pressure, which the medical consensus believes is causative for heart disease.

Ideally, the “smoking gun”...

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Why We Sleep Summary Chapters 9-11: The Benefits of Dreaming

Dreaming is a bizarre sensation. You’re unconscious, but you perceive intense vivid sensations and hallucinate things that aren’t there. You feel like you’re moving in the world, but your muscles have been forced to be limp. You remember faces and memories that you haven’t thought about for years, maybe decades. You had no control over your emotions, swinging from intense rage and jealousy to exuberance. Finally, when you woke up, you promptly forgot everything. If you experienced all of this while awake, you’d think you had a psychosis episode!

It’s not surprising then that dreaming has had a complicated history. In the ancient past, Egyptians and Greeks wondered if dreams were divine gifts from gods.

Freud helped dispel this, firmly centering it within the human brain. He considered dreams as expressions of repressed desires, and he built a psychological movement around interpreting dreams as such.

  • The critical flaw in Freudian analysis was its unprovability - the interpretation methods were so subjective that different approaches yielded different results, and there was no strict hypothesis that was testable.
  • Furthermore, the interpretations were as...

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Why We Sleep Summary Chapter 12: Sleep Disorders

We’ve talked before about how sleep deprivation causes disease. Now we’ll discuss sleep disorders, or primary issues with abnormal sleep, and their consequences.

Somnambulism / Sleepwalking

Sleepwalking is the act of walking and performing other behaviors while asleep. Automatic, nonconscious routines are executed, like brushing teeth or opening the refrigerator.

Sleepwalking happens during NREM sleep, and not REM dreaming sleep (like some think). Neurologically, sleepwalking is accompanied by an unexpected spike in nervous system activity, causing the person to be stuck somewhere between sleep and wakefulness.

Sleepwalking is more common in children than adults, for unknown reasons - possibly because kids spend more time in NREM sleep than adults do.

In one of the most extreme cases, a sleepwalker drove 14 miles to an in-laws’ home, stabbed the mother-in-law to death, and strangled the father-in-law (who survived). This person was later acquitted as not being control of his actions. This defense has been tried in later cases (most unsuccessfully).


Insomnia is defined as making enough time for sleeping, but having insufficient sleep...

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Why We Sleep Summary Chapter 13: What Stops You From Getting Good Sleep

Five influences have drastically changed how we sleep: caffeine, light, temperature, alcohol, and alarms.


This was already discussed in chapter 2. The tips, for good measure:

  • Caffeine is of course in coffee, some soft drinks, and some teas, but also chocolate.
  • Be careful when drinking decaf, as it apparently contains 15-30% of the caffeine in regular coffee - it’s nowhere near zero caffeine.
  • If you must have it, don’t drink it in the afternoon, and definitely not in the hours before sleep.


Light is a signal for the suprachiasmatic nucleus to regulate the circadian rhythm (by signaling to the pineal gland to secrete melatonin). In the natural world, when the sun goes down, there’s little light. But nowadays, artificial light bathes our homes and disrupts our circadian rhythm.

Any light is disruptive to the circadian rhythm. Electric light delays your 24-hour circadian rhythm by 2-3 hours each evening.

  • Even 8-10 lux (a measure of light intensity) delays melatonin release. A bedside lamp is 20-80 lux, and a typical living room is 200 lux, suppressing melatonin by 50%. (In comparison, the full moon only...

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Why We Sleep Summary Chapter 14: How to Get Better Sleep

Good Sleep Practices, Continued

In addition to avoiding all the problems from the last chapter (eg caffeine, alarms), here are more tips:

  • Keep the same waking and sleeping time each day. Erratic sleep schedules disrupt sleep quality.
  • Practice sleep hygiene - lower bedroom temperature, reduce noise, reduce light.
  • No alcohol, caffeine, exercise, or long naps before sleep.
  • Exercise seems to increase total sleep time and increase the quality of sleep.
    • This is more a chronic effect. This does not seem to act immediately on a day-to-day scale - exercise on one day doesn’t necessarily lead to better sleep that night. But worse sleep on one night does lead to worse exercise the following day.
  • Eat a normal diet (not severe caloric restriction of below 800 calories per day). Avoid very high carb diets (>70% of calories) since this decreases NREM and increases awakenings.

Sleeping Pills are Bad

Sleeping pills are typically sedatives that put the body into a state that doesn’t fully...

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Why We Sleep Summary Chapter 15: Society Causes Sleep Deprivation

Sleep deprivation goes far beyond our individual sleep practices. Our society has structurally locked in sleep deprivation in 4 ways.

Work Schedules Disrupt Sleep

The ethos at many companies sees sleep as an indulgence for the weak. They lionize the road warrior who fearlessly crosses time zones on tiny amounts of sleep and answers emails at 1AM. In their minds, more hours worked = more productivity.

This is short-sighted. The effects of sleep deprivation are costly to employers:

  • Lost productivity per sleep-deprived worker is in the thousands of dollars a year. The author argues insufficient sleep costs 2% of GDP.
    • In a natural experiment studying workers on opposite edges of a time zone, workers who obtained an hour of extra sleep earned 5% higher wages.
  • Sleep-deprived workers show bad traits:
    • Reduced work performance, creativity, motivation, social cohesion.
    • Increased risk-taking, impulsiveness, and desire to cheat.
  • Leaders who sleep worse are rated worse by their team and caused less engagement in their workers. This is visible day-by-day -- poor sleep one night is immediately seen as worse performance the...

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Why We Sleep Summary Chapter 16: Improving Sleep in Society

We’ve seen chronic sleep deprivation caused by a variety of factors, from the individual scaling up to the societal. The book ends with the author’s musings on how to improve sleep quality systemically:

  • Individual
    • Automated “internet of things” household that can automatically sense your circadian rhythm and personalize the temperature and lighting to maximize sleep.
      • Furthermore, if you have an upcoming disruption to your sleep schedule (like a flight), it can adjust your circadian rhythm to smoothen the transition.
    • Sophisticated body trackers that record a host of factors -- sleep, physical activity, light exposure, temperature, heart rate, mood, happiness, social performance, productivity -- and shows how your sleep correlates with...

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Table of Contents

  • 1-Page Summary
  • Chapter 2: Your Daily Sleep Rhythm
  • Chapter 3: Sleep Cycles within a Night
  • Chapter 4: Interesting Facts About Sleep
  • Chapter 5: How Sleep Changes from Childhood to Adulthood
  • Chapter 6: How Sleep Benefits the Brain
  • Chapter 7: How Sleep Deprivation Harms the Brain
  • Chapter 8: How Sleep Deprivation Harms the Body
  • Chapters 9-11: The Benefits of Dreaming
  • Chapter 12: Sleep Disorders
  • Chapter 13: What Stops You From Getting Good Sleep
  • Chapter 14: How to Get Better Sleep
  • Chapter 15: Society Causes Sleep Deprivation
  • Chapter 16: Improving Sleep in Society