PDF Summary:Why We Sleep, by Matthew Walker
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1-Page PDF Summary of Why We Sleep
You’re probably getting less sleep than you should be. Many people are more chronically sleep-deprived than they realize, and the punishments for this are severe—reduced productivity and happiness, and an increased risk of a panel of diseases. In Why We Sleep, “sleep diplomat” Matthew Walker, Ph.D. unlocks some of the mysteries of sleep based on research from both in and outside his Center for Human Sleep Science.
In this guide, we’ll explore how sleep happens, its major benefits, and the best ways to get better sleep. We’ll also discuss sleep research from other experts as well as new findings that emerged after the book was published.
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2. Sleep deprivation worsens emotional control. Walker says that when you’re sleep deprived, your amygdala (the part of your brain that controls emotion) can run amok, leading to 60% more emotional reactivity. Sleep disruption is a common symptom of all mood disorders. However, sleep deprivation actually makes one-third of depression patients feel better. (Shortform note: Guzey notes that Walker downplays the benefits of sleep deprivation—studies suggest that it’s beneficial to about 45-50% of patients with depression. Walker clarifies that these findings emerged after the book was written.)
3. Sleep deprivation may contribute to Alzheimer’s. Sleep loss may disrupt memory formation as well as the glymphatic cleanup system, which clears out Alzheimer’s-associated plaques. (Shortform note: A study suggests that sleeping on your side can help decrease the chances of developing Alzheimer’s and other neurological diseases.)
Diseases Linked to Sleep Deprivation
In addition to the damage it causes the brain, sleep deprivation disrupts the normal function of many physiological processes, likely contributing to the following:
- Heart disease
- Diabetes
- Obesity and weight gain
- Reduced reproduction (by affecting hormones and attractiveness)
- Some cancers
- Aging
- Reduced athletic performance
- Death
A Note on the Studies in Why We Sleep
Many of the population studies cited in Why We Sleep are correlational—for example, their results show that people who sleep less are more likely to have heart disease, after controlling for many other factors. But the causation of these results is unclear—some other factors that predispose people to heart disease (like a high baseline level of stress) could also reduce sleep.
To address this, the experimental studies Walker cites attempt to link lack of sleep to a middle physiological state, which itself is causative for the disease. For instance, a lack of sleep increases blood pressure, which the medical consensus believes is causative for heart disease.
Ideally, the “smoking gun” experiment would be to randomize people into normal-sleep and low-sleep groups for years, then observe the rate of disease. However, this is impractical (it’s hard to run very long studies like this and impossible to double-blind) and likely unethical (if low sleep is already believed to cause severe disease).
Part 3: The Science of Dreams
Most vivid dreaming happens during REM sleep. Walker says that your visual, motor, memory, and emotional areas of the brain are active. Your prefrontal cortex (governing rationality) is muted. Some people are even capable of lucid dreaming, meaning they’re able to voluntarily control their actions within their dream. (Shortform note: One study found that you can increase your chances of having lucid dreams by combining three techniques: reality testing, breaking up your sleep, and mnemonic induction of lucid dreams.)
Benefits of Dreaming and REM Sleep
Walker says there are three ways dreaming and REM sleep are good for you:
- REM dreaming blunts emotional pain from memories—the brain seems to reprocess upsetting memories and emotional themes in a way that retains the useful lessons while lessening the visceral emotional pain. (Shortform note: Walker attributes this only to REM sleep, but neuroscientist Rebecca Spencer posits that NREM sleep also plays a role.)
- REM sleep increases your understanding of other people’s emotions—sleep-deprived people more often interpret faces as hostile and aggressive. (Shortform note: This works both ways: While sleep-deprived individuals view other people more negatively, other people also view sleep-deprived people as more unpleasant.)
- REM sleep creates novel connections and a higher-level comprehension of ideas, and increases your ability to solve creative problems. (Shortform note: In fact, it may be possible to manipulate your brain to solve problems during sleep.)
Part 4: The Current State of Sleep
Walker finishes by covering sleep disorders, sleep disruptors, and ways to get better sleep.
Sleep Disorders
He explains that there are three sleep disorders that people commonly experience:
1. Somnambulism (sleepwalking)—the act of walking and performing other behaviors while asleep. It happens during NREM sleep. (Shortform note: A 2021 study suggests that men who sleepwalk may have a higher risk of developing Parkinson’s disease. The two share a common neural pathway and both involve involuntary movements, confusion, and amnesia.)
2. Insomnia—defined as making enough time for sleeping, but having insufficient sleep quantity or quality, for more than three months. The most common triggers are emotional concerns or distress. (Shortform note: Researchers suggest that physical and emotional stress—from discrimination in the workplace and financial pressure due to unemployment, for example—may be the reason behind the sleep disparity between people of color and white people.)
3. Narcolepsy—a not-awake, not-asleep state marked by three symptoms: sudden bouts of extreme sleepiness, sleep paralysis (waking up in REM sleep during muscle atonia), and cataplexy (sudden loss of muscle control while awake). (Shortform note: It can be hard to diagnose narcolepsy because its symptoms overlap with the symptoms of depression, hypothyroidism, sleep apnea, and epilepsy, among other conditions.)
What Stops You From Getting Good Sleep
Walker names jet lag as an increasingly common sleep disruptor. It affects travelers by disturbing the circadian rhythm. He says it can take you up to 10 days to readjust to a 10-hour time difference. (Shortform note: There are several ways to minimize the effects of jet lag, such as hydrating adequately and exposing yourself to natural light at your destination.)
He says that even those who aren’t traveling face five major influences that have drastically changed how we sleep:
1. Caffeine—blocks adenosine receptors, thus reducing how much you feel the desire to sleep. (Shortform note: If you can’t shake the habit of drinking a hot beverage late in the afternoon or evening, try herbal teas that may help promote sleep and relaxation.)
2. Light—nowadays, artificial light constantly fills our homes and disrupts our circadian rhythm. Blue light is most problematic, suppressing melatonin at twice the levels of warm light. (Shortform note: Blue light may have an even bigger impact on children, as they’re more sensitive to light and have bigger pupils.)
3. Constant temperature—in modern times, thermostats homogenize temperatures, suppressing the biological systems that use temperature changes throughout the day as sleep cues. (Shortform note: If it’s too hot and you have no access to air-conditioning, cool your body temperature by avoiding exercising at night, avoiding anything spicy, and stashing your pillowcases in the fridge.)
4. Alcohol—a sedative that causes what appears to be sleep but is really more like anesthesia. It causes you to wake up throughout the night and prevents you from getting REM sleep. Walker encourages total abstinence from alcohol. (Shortform note: A less puritanical approach comes from The Sleep Foundation, who recommend that you stop drinking alcohol at least four hours before bedtime.)
5. Alarms—cause acute stress responses when you wake up, spiking your cortisol levels, heart rate, and blood pressure. (Shortform note: One study suggests that changing your alarm from a jarring sound to an upbeat song can help combat sleep inertia, that disoriented state you’re in upon waking up.)
How to Get Better Sleep
Walker provides a number of tips on how you can start getting better, less interrupted sleep.
- 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.
- Avoid alcohol, caffeine, exercise, or long naps before sleep.
- Get some exercise, which may increase total sleep time and increase quality of sleep. Exercising has more of a chronic effect, meaning it helps in the long run and doesn’t take effect 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.
- Avoid sleeping pills—they’re no better than a placebo.
- For those with insomnia, try cognitive behavioral therapy, which has been shown to be more effective than sleeping pills.
More Tips to Help You Sleep
In The Sleep Revolution: Transforming Your Life, One Night at a Time, Arianna Huffington devotes a chapter to sleep tips, tools, and techniques. While there are many overlaps with While We Sleep (such as limiting blue light, avoiding alcohol, and getting the temperature right), she offers some additional tips that Walker doesn’t mention:
Try acupuncture. According to a study, this centuries-old practice had a positive effect on 93% of insomnia patients.
Sip (or sniff) some lavender. Studies suggest that the herb has a relaxing effect, which can set the stage for a good night’s sleep. Huffington writes that in Germany, lavender tea is an approved treatment for insomnia. You can also try spritzing some lavender onto your pajamas or sheets.
Empty your mind. To help you reduce anxiety-producing thoughts, try doing a “mind dump” before bed: Write down your to-do list for the next day so that your thoughts won’t keep you up at night.
Improving Sleep in Society
Walker argues that sleep deprivation goes far beyond individual sleep practices. He says that our society has structurally locked in sleep deprivation in two ways: First, work schedules disrupt sleep. Companies associate hours worked with productivity and tend to see sleep as an indulgence of the weak. Second, school schedules disrupt sleep: Early start times disrupt children’s circadian rhythms. Walker offers ways to improve sleep quality in society:
- Employers should focus less on hours worked and instead implement flexible hours to suit personal circadian rhythms. They can even incentivize sleep with vacation days or bonuses.
- Use sleep technology to improve sleep tracking and help you adjust your circadian rhythm when needed.
- Educate the general public about the importance of sleep, in the same way schools have educational programs about diet and drugs.
- Promote sleep hygiene for hospital patients—hospitals can replace their harsh lighting and find ways to minimize beeping noises at night.
(Shortform note: The sleep landscape continues to change with technological advancements that we can use to improve our sleep. In 2021, the FDA approved SleepCogni for medical use. Clinical trials have shown that SleepCogni, a data-supported device, can reduce insomnia in just seven days. In the hospitality industry, where sleep is an essential part of a hotel guest’s stay, a scientist predicts some ways in which the experience of sleep will change, including bed covers and sheets made of high-tech thermal regulation fabrics and pillows that can detect sleep activity.)
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PDF Summary Shortform Introduction
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Walker’s official website states that he earned his Ph.D. in neurophysiology from the Medical Research Council (MRC) in London. However, in a blog post critiquing Walker’s book, researcher Alexey Guzey points out that the MRC does not grant Ph.D. degrees. Walker’s Wikipedia page states that he received his Ph.D. from Newcastle University, and that his research was funded by the MRC. It is unclear why there is a discrepancy in information.
Connect with Dr. Matthew Walker:
The Book’s Publication
Why We Sleep: Unlocking the Power of Sleep and Dreams, published in 2017 by Simon & Schuster, is Walker’s first book. It became a New York Times Best Seller and landed at the top of the Sunday Times Bestseller list in the U.K.
The Book’s Context
Historical Context
Walker sees...
PDF Summary Part 1: How Sleep Works | Chapter 2: Your Daily Sleep Rhythm
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Circadian rhythms vary from person to person, dictating when they naturally wake up and have maximum energy. Walker says that this proves that the idea of “morning people” and “night owls” is real.
- Whether you’re a morning or night person strongly depends on your genetics.
- Humans evolved with this variation because having a mixture of morning people and night owls allows a population to reduce its vulnerability. For example, as morning people go to sleep earlier (say at 10PM), night owls can keep up the watch. Then as night owls get tired (say around 4AM), the morning people are starting to wake.
- Walker argues that in modern times, the night owls are heavily punished, since early start times at work force night owls to sleep and wake up earlier than their bodies are optimized to. This reduces performance in the mornings. Furthermore, by the time night owls peak in the afternoon, the work day has already ended.
Can a Night Owl Become a Morning Person?
It’s very hard to change your chronotype (your biological propensity to sleep at particular hours), but given the...
PDF Summary Chapters 3-5: The Human Sleep Cycle
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On the graph, notice that not all sleep cycles look the same. As the sleep progresses through the night, a greater fraction of each cycle is spent in REM sleep.
Why would the sleep cycles be unbalanced in this way? Why not just have all sleep cycles look the same, with 80% in NREM and 20% in REM?
Walker hypothesizes that it’s similar to tidying up a house: First, NREM declutters, then REM gets into the finer points of cleaning and organizing.
(Shortform note: One way to think about this is that an animal might be interrupted in the middle of the night. If an animal could only sleep three hours in one night, it’d make sense for the more critical functions to be performed first, with the later functions being a luxury if the animal could sleep a full night. This may suggest that NREM performs a more vital function for survival.)
Also beware of what this means for cutting your sleep short. If you normally sleep eight hours, and one night you have to cut your sleep to six hours, then you’re not just losing 25% of sleep—you might be losing 60-90% of your REM sleep.
Likewise, Walker warns that going to sleep later than usual might cut short your NREM sleep.
(Shortform note:...
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Learn more about our summaries →PDF Summary Part 2: The Importance of Sleep | Chapter 6: How Sleep Benefits the Brain
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(Shortform note: A 2019 study on mice found that melanin concentrating hormone (MCH) neurons helped the brain forget unimportant information during REM sleep. This could help researchers better understand—and possibly come up with treatments for—memory-related diseases and disorders.)
3) Sleep Increases “Muscle Memory” or Motor Task Proficiency
You might struggle with a motor task (like playing a tough sequence on a piano), but after sleeping, be able to play it flawlessly. Sleep seems to transfer motor memories to subconscious habits.
- Motor memory is associated with stage 2 NREM, which is concentrated in the last cycle of sleep.
(Shortform note: While Walker writes that motor memory is connected to NREM sleep, the Division of Sleep Medicine at Harvard School suggests that motor learning is linked to REM sleep. They acknowledge that researchers have varying views when it comes to the sleep stages linked to memory, but that the overall evidence suggests that sleep in general is...
PDF Summary Chapter 7: How Sleep Deprivation Harms the Brain
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The High Risk of Sleep Deprivation
Walker warns that the combination of reduced concentration and an inflated sense of your capabilities in a sleep-deprived state is especially harmful during high-risk activities, like driving. To put the risk into perspective:
- Driving after having slept less than four hours increases crash risk 11.5 times.
- Being awake for 19 hours (being past your bedtime by three 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.
(Shortform note: In the original text, Walker wrote that there are more vehicular accidents caused by drowsy driving than by alcohol and drugs combined. He has since removed this vague claim from the book and revised it to include statistics, saying it’s hard to objectively quantify the number of accidents due to drowsy driving versus driving under the influence.)
Why do sleep deficits 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 a car going 60...
PDF Summary Chapter 8: How Sleep Deprivation Harms the Body
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1) Heart Disease
Walker claims that sleep deprivation has a number of effects related to cardiovascular disease:
- It activates the sympathetic nervous system, leading to:
- Increased heart rate
- Increased vasoconstriction, and subsequently, increased blood pressure
- Increased cortisol (stress hormone)
- Increased atherosclerosis (especially of coronary arteries)
- Through hormone signaling, it decreases HDL (good cholesterol) and growth hormone (which promotes recovery of blood vessel endothelium)
A population study showed that shorter sleep was associated with a 45% increased risk of developing heart disease.
He notes an interesting finding: daylight savings time is a natural sleep experiment that typically increases or decreases sleep by one hour. When the clock moves forward and the population gets one less hour of sleep, there is a significant spike in heart attacks and the number of traffic accidents.
(Shortform note: Guzey writes that [Walker seems to misrepresent studies on the relationship between sleep and cardiovascular...
PDF Summary Part 3: The Science of Dreams | Chapters 9-11: The Benefits of Dreaming
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Two of the newer theories that emerged in 2021 are:
Network exploration to understand possibilities (NEXTUP). In When Brains Dream, sleep researchers Antonio Zadra and Robert Stickgold theorize that dreams, and not just the sleep state, are important for memory consolidation, allowing dreamers to create links between themes and memories and using narrative structures to help the brain figure out which of these associations may be useful.
Overfitted brain hypothesis. Another theory suggests that dreams in all their strangeness give the human brain a more well-rounded view of the world. This was inspired by artificial intelligence, which tends to have a bias toward the data it’s trained on. The human brain is trained to process similar data every day.
The Science of Dreaming
Most vivid dreaming happens during REM sleep (though NREM sleep has some vague non-vivid dreaming, like “I was thinking about clouds”).
During REM...
PDF Summary Part 4: The Current State of Sleep | Chapter 12: Sleep Disorders
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- Instead of jolting an actively sleepwalking person awake, gently guide them back to their bed.
Insomnia
Walker defines insomnia as making enough time for sleeping, but having insufficient sleep quantity or quality, for more than three months. Symptoms include difficulty falling asleep, waking up in the middle of night, and feeling unrefreshed in the morning.
He says that when they do sleep, insomniacs have more fragmented REM sleep and shallower brain waves in NREM.
One out of nine people suffer from insomnia. It’s twice as common in women than men, and more common in Black and Hispanic people than white, for unknown reasons.
The most common triggers of insomnia are emotional concerns or distress. The biological cause is linked to an overactive sympathetic nervous system, which raises body temperature and levels of cortisol and epinephrine. In turn, the thalamus, hippocampus, and amygdala all remain more active than in normal sleeping patients.
(Shortform note: Researchers suggest that physical and emotional stress may be the reason behind the sleep disparity...
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PDF Summary Chapter 13: What Stops You From Getting Good Sleep
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- Keep in mind that caffeine doesn’t just come from coffee, so you may be ingesting caffeine without realizing it. It’s also in some soft drinks, some teas, and chocolate. (Shortform note: Black and green teas contain nearly a third of the caffeine in coffee. Another source of caffeine is energy drinks, which not only contain nearly the same amount of caffeine as coffee but also have high amounts of sugar and artificial additives.)
- Caffeine exposure during childhood could reduce NREM sleep, delaying brain maturation and learning. (Shortform note: The U.S. doesn’t have any guidelines for children’s caffeine consumption, but Canada recommends a range from 45 mg or half a cup of coffee for those aged four to six, to 85-100 mg for adolescents.)
Tip to Manage Caffeine Intake
Walker says that if you must have caffeine, don’t drink it in the afternoon, and definitely not in the hours before sleep.
(Shortform note: If you can’t shake the habit of drinking a hot beverage late in the afternoon or in the evening, try...
PDF Summary Chapter 14: How to Get Better Sleep
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According to Walker, sleeping pills are no better than placebo when it’s time to fall asleep (even though self-reported satisfaction is higher). The lower quality of sleep causes daytime sleepiness.
Additionally, sleeping pills can kick off a heavily medicated vicious cycle:
- Poor sleep practices or stress reduces sleep.
- Taking sleeping pills causes next-day drowsiness.
- Caffeine use and naps reduce drowsiness, but also reduce ability to sleep at night, causing more sleeping pill usage.
- Tolerance of sleeping pills causes withdrawal insomnia when stopped, thus maintaining the habit.
Walker cites population studies that show that sleeping pills increase mortality in a dose-dependent way. Suggestive causes:
- Increased risk of car accidents—possibly from grogginess, a side effect of some sleeping pills
- Increased risk of cancer—though there is only evidence of correlation, not causation, between pill use and cancer.
- Increased infection risk (especially prevalent in elderly subjects)—pill-induced sleep may not provide the immunity benefits of natural sleep.
(Shortform note: We can’t rule out that upstream circumstances that disrupt sleeping and thus...
PDF Summary Chapter 15: Society Causes Sleep Deprivation
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Solutions for Employers
- Focus less on hours worked, and more on real productivity and output.
- Let people have flexible work hours to suit personal circadian rhythms. Don’t punish night owls more than morning people.
- Add nap pods and adaptive office lighting to promote better sleep.
How the Covid-19 Pandemic Changed Sleep
During the Covid-19 pandemic, many people had more relaxed work schedules, as they were working from home and didn’t have to spend time commuting to and from the office. The combination of more time at home plus the pandemic environment changed sleep on three levels, according to Walker:
- Amount—Walker says that a sleep tracking company reported that total sleep time increased by 20%. He also cites two studies that support the finding that people were sleeping more during lockdown. One finding was that sleep schedules across weekdays and weekends were more consistent. Walker thinks this was because no commute meant extra sleep time in the morning and at night. They also didn’t have to get up too early to get...
PDF Summary Chapter 16: Improving Sleep in Society
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Educational
- Walker argues that sleep should be a mandatory subject in physical education (like drugs and diet).
- Have predictive tools that show the costs to health and income if you continue your poor sleep habits.
- Instruct the populace on sleepy driving as much as drunk driving.
(Shortform note: In Switch, the authors cite an example of how the general public learned about the concept of designated drivers: A Harvard professor got more than 160 TV programs to work the topic into their shows—as part of the script or as a poster in the background, for example. Within three years, 90% of Americans knew about designated driving, and there was also a decrease in alcohol-related crashes. A similar tactic might make the public more aware of drowsy driving.)
Organizational
- Companies should be more flexible about work hours, allow naps, and de-emphasize hours worked in favor of real productivity. (Shortform note: While [a flexible work arrangement leads to happier and more productive...