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Why We Sleep by Matthew Walker - Summary and Book Notes

Update: it's been brought to my attention that some of the scientific claims made in this book may be incorrect or misrepresented by the author. When reading any book, it's important to read with a critical eye. Read more on this discussion here.

Everyone knows we should 'get enough sleep' but stumbles to work with 6 or fewer hours of shuteye. Matthew Walker makes a hugely damning case against inadequate sleep.

This is a summary of a great Tree Book. Tree books are books that lay out a framework of ideas. Here Walker covers a wide range of topics surrounding sleep.

Review -- Why We Sleep by Matthew Walker

Why We Sleep by Matthew Walker

I highly recommend for people:

  • who 'need' a coffee to get going in the morning
  • who are struggling with low energy levels
  • are curious about the role of dreaming on memory and creativity

Skip this book if you:

  • already have great sleep hygiene and don't need more convincing

Buy Why We Sleep on Amazon

Quick Summary of Why We Sleep

Lack of sleep is bad for your physical health, your mental health, your longevity, your ability to learn, your creativity, your emotional control, and more.

Our modern environment is terrible for getting good sleep. Bright lights, electronic devices, late-night weekends and drinking all impair your ability to sleep well.

Every single part of your body is enhanced by sleep. This is not hyperbole.

Two forces drive how sleepy you feel:

  1. Circadian Rhythm
  2. Sleep Pressure (Adenosine)

Two types of sleep:

  1. Non-Rapid-Eye-Movement (NREM)
  2. Rapid-Eye-Movement (REM)

Alcohol suppresses REM sleep, and causes sedation, not natural sleep.

Stop taking sleeping pills: No past or current sleeping medications on the legal (or illegal) market induce natural sleep.

12 Tips for Healthy Sleep from Matthew Walker (click to jump to section)

Why We Sleep Book Notes

The following are rough notes I took while reading. These are mostly paraphrased or quoted directly from the book. I've rearranged some ideas from their original book order to better fit themes.

Our society is not conducive to sleep

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

Five key factors have powerfully changed how much and how well we sleep:

  1. Constant electric light as well as LED light
  2. Regularized temperature
  3. Caffeine
  4. Alcohol
  5. A legacy of punching time cards

Artificial evening light, even that of modest strength, will fool your suprachiasmatic nucleus (Circadian Rhythm regulator) into believing the sun has not yet set.

Electric light winds back your internal twenty-four-hour clock two to three hours each evening, on average.

Artificial evening and nighttime light can masquerade as sleep-onset insomnia—the inability to begin sleeping soon after getting into bed.

The light receptors in the eye that communicate “daytime” to the suprachiasmatic nucleus are most sensitive to short-wavelength light within the blue spectrum—the exact sweet spot where blue LEDs are most powerful.

We stare at LED-powered laptop screens, smartphones, and tablets each night, sometimes for many hours, often with these devices just feet or even inches away from our retinas.

Compared to reading a printed book, reading on an iPad suppressed melatonin release by over 50 percent at night. Indeed, iPad reading delayed the rise of melatonin by up to three hours, relative to the natural rise in these same individuals when reading a printed book.

Ways to reduce the impact of lights on your sleep schedule:

  • Lower light in rooms where you spend evening hours
  • Avoid powerful overhead lighting
  • Wear yellow-tinted glasses
  • Install software like flux to reduce blue light on screens at night
  • Maintain complete darkness at night with blackout curtains or eyemasks
  • Cool room, hot bath just before bed
  • Wake up with first alarm, don't snooze (repeated fight-or-flight response)
  • Waking up at the same time of day, every day, no matter if it is the week or weekend is a good recommendation for maintaining a stable sleep schedule if you are having difficulty with sleep.

Lack of sleep is bad for your physical health

I was once fond of saying, “Sleep is the third pillar of good health, alongside diet and exercise.” I have changed my tune. Sleep is more than a pillar; it is the foundation on which the other two health bastions sit.

The shorter your sleep, the shorter your life.

Heart health

Progressively shorter sleep was associated with a 45 percent increased risk of developing and/or dying from coronary heart disease

Those sleeping six hours or less were 400 to 500 percent more likely to suffer one or more cardiac arrests than those sleeping more than six hours.

Deficient sleep puts your body into the stressful fight-or-flight state

One night of modest sleep reduction—even just one or two hours—will promptly speed the contracting rate of a person’s heart, hour upon hour, and significantly increase the systolic blood pressure within their vasculature.

In the Northern Hemisphere, the switch to daylight savings time in March results in most people losing an hour of sleep opportunity...this seemingly trivial sleep reduction comes with a frightening spike in heart attacks the following day.

When the clocks move forward and we gain an hour of sleep opportunity time, rates of heart attacks plummet the day after.

There are far higher rates of type 2 diabetes among individuals that reported sleeping less than six hours a night routinely.


Inadequate sleep decreased concentrations of the satiety-signaling hormone leptin and increased levels of the hunger-instigating hormone ghrelin.

Cravings for carbohydrates and salty snacks increase, while cravings for protein-rich foods do not.

Sleep-depriving an individual for twenty-four hours straight and they will only burn an extra 147 calories (less than 3 oreos)

If you're on a diet, lack of sleep means more of the weight lost will be comprised of lean body tissue (muscle). If you're aiming to bulk up, lack of sleep means more of the weight gained will be comprised of fat.

Sex Hormones

The size of the hormonal blunting effect is so large that [lack of sleep] effectively “ages” a man by ten to fifteen years in terms of testosterone virility.

Routinely sleeping less than six hours a night results in a 20 percent drop in follicular-releasing hormone (regulates ovary function) in women

Beauty sleep is real: faces pictured after one night of short sleep were rated as looking more fatigued, less healthy, and significantly less attractive

Immune system

A single night of four hours of sleep swept away 70 percent of the natural killer cells circulating in the immune system, relative to a full eight-hour night of sleep.


Sleeping six hours or less was associated with a 40 percent increased risk of developing cancer, relative to those sleeping seven hours a night or more.


After one week of subtly reduced sleep, the activity of a hefty 711 genes was distorted, relative to the genetic activity profile of these very same individuals when they were obtaining eight and a half hours of sleep for a week.


A chronic lack of sleep across the season predicted a massively higher risk of injury (figure 10).

Chart showing decrease in injury rates with increase in sleep times


Medical errors are the third-leading cause of death among Americans after heart attacks and cancer.

Throughout the course of their residency, one in five medical residents will make a sleepless-related medical error that causes significant, liable harm to a patient. One in twenty residents will kill a patient due to a lack of sleep.

When a sleep-deprived resident finishes a long shift, such as a stint in the ER trying to save victims of car accidents, and then gets into their own car to drive home, their chances of being involved in a motor vehicle accident are increased by 168 percent because of fatigue.

Lack of sleep is bad for your mental health

Dreaming helps soothe painful memories and spark creativity

REM-sleep dreaming offers a form of overnight therapy. That is, REM-sleep dreaming takes the painful sting out of difficult, even traumatic, emotional episodes you have experienced during the day, offering emotional resolution when you awake the next morning.

Concentrations of a key stress-related chemical called noradrenaline are completely shut off within your brain when you enter this dreaming sleep state. In fact, REM sleep is the only time during the twenty-four-hour period when your brain is completely devoid of this anxiety-triggering molecule.

Lack of sleep leads to lowered emotional control

Parents know that a lack of sleep leads to bad mood and emotional reactivity. "He hasn't had his nap yet"

With a full night of plentiful sleep, we have a balanced mix between our emotional gas pedal (amygdala) and brake (prefrontal cortex). Without sleep, however, the strong coupling between these two brain regions is lost.

The under-slept brain swings excessively to both extremes of emotional valence, positive and negative.

A dream-starved brain cannot accurately decode facial expressions, which become distorted. the sleep-deprived participants slipped into a default of fear bias, believing even gentle- or somewhat friendly looking faces were menacing.

You're less likely to be ethical when sleep deprived. The less an individual sleeps, the more likely they are to create fake receipts and reimbursement claims

Ethical deviance linked to a lack of sleep also weasels its way onto the work stage in a different guise, called social loafing.


Sleep disruption and Alzheimer’s disease interact in a self-fulfilling, negative spiral that can initiate and/or accelerate the condition.

Alzheimer's is associated with the buildup of a protein called beta-amyloid. The glymphatic system collects and removes dangerous metabolic contaminants like beta-amyloid. Especially at night during deep NREM sleep. Associated with the pulsing rhythm of deep NREM sleep comes a ten- to twentyfold increase in effluent expulsion from the brain. In what can be described as a nighttime power cleanse, the purifying work of the glymphatic system is accomplished by cerebrospinal fluid that bathes the brain.

Amyloid plaques build up in the brain, especially in deep-sleep-generating regions; worsening your ability to get deep sleep, causing more amyloid buildup -> negative spiral

Wakefulness is low-level brain damage, while sleep is neurological sanitation.

Getting too little sleep across the adult life span will significantly raise your risk of developing Alzheimer’s disease.

Psychiatric conditions

Should you improve sleep quality in patients suffering from several psychiatric conditions using ... cognitive behavioral therapy for insomnia (CBT-I), you can improve symptom severity and remission rates.

(lack of sleep is not necessarily cause of psychiatric conditions, psychiatric conditions are almost always accompanied by impaired sleep quality)


The two most common triggers of chronic insomnia are psychological: (1) emotional concerns, or worry, and (2) emotional distress, or anxiety.

One of the few times that we stop our persistent informational consumption and inwardly reflect is when our heads hit the pillow. There is no worse time to consciously do this.

Every part of your body is enhanced by sleep

When sleep was poor the night prior, exercise intensity and duration were far worse the following day. When sleep was sound, levels of physical exertion were powerfully maximal the next day.

Poor Sleep has large societal costs

Inadequate sleep costs America and Japan $411 billion and $138 billion each year, respectively.

Lack of sleep is deadly for drivers

Every hour, someone dies in a traffic accident in the US due to a fatigue-related error.

The result is worse than a drunk driver. The drunk driver is slow to react, but at least they react. During a microsleep, drivers will make no reaction at all (no braking, no swerving to avoid), leading to worse outcomes

After the first night of no sleep at all, their lapses in concentration (missed responses) increased by over 400 percent.

Ten days of six hours of sleep a night was all it took to become as impaired in performance as going without sleep for twenty-four hours straight.

After being awake for nineteen hours, people who were sleep-deprived were as cognitively impaired as those who were legally drunk.

You do not know how sleep-deprived you are when you are sleep-deprived

How Sleep works:

Two forces drive how sleepy you feel:

  1. Circadian Rhythm
  2. Sleep Pressure (Adenosine)

Two types of sleep:

1. Non-Rapid-Eye-Movement (NREM)

  • Dominates the first half of the night
  • Helps shuttle information to long term memory
  • Removes out unneeded connections
  • Helps turn new physical skills into unconscious motor memory

Go to bed late: lose NREM, lose storage of new information.

For fact-based information—or what most of us think of as textbook-type learning, such as memorizing someone’s name, a new phone number, or where you parked your car—a region of the brain called the hippocampus helps apprehend these passing experiences and binds their details together.

The hippocampus has a limited storage capacity, and NREM sleep helps move information from the limited hippocampus to longer term memory.

Sleep for memory consolidation is an all-or-nothing event.

The “ah yes, now I remember” sensation that you may have experienced after a good night of sleep [is a result of NREM].

Practice does not make perfect. It is practice, followed by a night of sleep, that leads to perfection.

Rather than a transfer from short- to long-term memory required for saving facts, the motor memories had been shifted over to brain circuits that operate below the level of consciousness [During NREM sleep]. As a result, those skill actions were now instinctual habits.

2. Rapid-Eye-Movement (REM)

  • Dominates the later half of the night
  • Connects new experiences to distant, unrelated information. This builds creativity and understanding
  • Strengthens connections
  • Helps forget painful emotions surrounding difficult memories

Wake up early: lose REM sleep, lose creativity.

Only birds and mammals have REM sleep.


Sleep provides a nighttime theater in which your brain tests out and builds connections between vast stores of information. This task is accomplished using a bizarre algorithm that is biased toward seeking out the most distant, nonobvious associations, rather like a backward Google search. rem

REM sleep is capable of creating abstract overarching knowledge and super-ordinate concepts out of sets of information.

REM sleep, and the act of dreaming, takes that which we have learned in one experience setting and seeks to apply it to others stored in memory.

It postulated that the process of REM-sleep dreaming accomplishes two critical goals:

  1. sleeping to remember the details of those valuable, salient experiences, integrating them with existing knowledge and putting them into autobiographical perspective, yet
  2. sleeping to forget, or dissolve, the visceral, painful emotional charge that had previously been wrapped around those memories.

Alcohol and REM

Alcohol strongly suppresses REM sleep. Alcohol sedates you out of wakefulness, but it does not induce natural sleep

Those who had their sleep laced with alcohol on the first night after learning suffered what can conservatively be described as partial amnesia seven days later, forgetting more than 50 percent of all that original knowledge.

No past or current sleeping medications on the legal (or illegal) market induce natural sleep.

School Start times

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.

It is the lack of REM sleep—that critical stage occurring in the final hours of sleep that we strip from our children and teenagers by way of early school start times—that creates the difference between a stable and unstable mental state.

Research findings have also revealed that increasing sleep by way of delayed school start times wonderfully increases class attendance, reduces behavioral and psychological problems, and decreases substance and alcohol use. In addition, later start times beneficially mean a later finish time. This protects many teens from the well-researched “danger window” between three and six p.m., when schools finish but before parents return home.


Older adults need the same amount of sleep, they just have trouble generating it

Not all medical problems of aging are attributable to poor sleep. But far more of our age-related physical and mental health ailments are related to sleep impairment than either we, or many doctors, truly realize or treat seriously.

Family members observe these daytime features in older relatives and jump to a diagnosis of dementia, overlooking the possibility that bad sleep is an equally likely cause.


Even daytime naps as short as twenty minutes can offer a memory consolidation advantage, so long as they contain enough NREM sleep.

The practice of biphasic sleep is not cultural in origin, however. It is deeply biological. All humans, irrespective of culture or geographical location, have a genetically hardwired dip in alertness that occurs in the midafternoon hours.

12 Tips for Healthy Sleep from Matthew Walker

1. Stick to a sleep schedule. Go to bed and wake up at the same time each day.

2. Try to exercise at least thirty minutes on most days but not later than two to three hours before your bedtime.

3. Avoid caffeine and nicotine.

4. Avoid alcoholic drinks before bed.

5. Avoid large meals and beverages late at night.

6. If possible, avoid medicines that delay or disrupt your sleep.

7. Don’t take naps after 3 p.m.

8. Relax before bed. Don’t over-schedule your day so that no time is left for unwinding.

9. Take a hot bath before bed. The drop in body temperature after getting out of the bath may help you feel sleepy.

10. Dark bedroom, cool bedroom, gadget-free bedroom. Get rid of anything in your bedroom that might distract you from sleep, such as noises, bright lights, an uncomfortable bed, or warm temperatures.

11. Have the right sunlight exposure. Daylight is key to regulating daily sleep patterns. Try to get outside in natural sunlight for at least thirty minutes each day. If possible, wake up with the sun or use very bright lights in the morning.

12. Don’t lie in bed awake. If you find yourself still awake after staying in bed for more than twenty minutes or if you are starting to feel anxious or worried, get up and do some relaxing activity until you feel sleepy.

The book also gives a great overview of how each type of sleep works. It's a fascinating read, but I've largely skipped it in this summary in favor of more practical takeaways.

Buy Why We Sleep on Amazon.

Further Reading