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Book Summary: Why We Sleep – Unlocking the Power of Sleep and Dreams

During sleep, your brain transition between three types of sleep: deep sleep, light sleep, and REM sleep.

During sleep, your brain transition between three types of sleep: deep sleep, light sleep, and REM sleep.

The internet never sleeps. And as the world, in its connectivity, comes to resemble the net, with everyone and everything linked by phones and computers and tablets, sleep may come to be seen as superfluous – a time-wasting activity of the pre-digital past.

But, till that dismal day arrives, we shouldn’t underestimate the power and importance of sleep.

As you’ll learn in this book summary, sleep is crucial to our well-being, as well as our ability to function – and being deprived of it can have both personal and societal consequences. Yes, the modern world is wakeful and fast-paced, but we humans must get enough sleep to survive and thrive in it.

In this summary of Why We Sleep by Matthew Walker, you’ll also learn

  • which animal never closes its eyes;
  • how deadly sleepwalking can be; and
  • how drowsiness compares to drunkenness.

Book Summary: Why We Sleep - Unlocking the Power of Sleep and Dreams

Deep Sleep

During the day, your hippocampus (a finger‐shaped region in the middle of your brain) temporarily stores information, like names or the steps of a new work procedure. During deep sleep, your mind transports data from the hippocampus to permanent storage locations in the brain, like a mail delivery service transporting packages from a mailroom to homes around a city.

If you decide to stay up late and skip out on the first two hours of your regular sleep schedule, you’ll miss most of your deep sleep and fail to store important information in your long‐term memory.

Light Sleep

Light sleep acts like the mailroom cleaning staff – it clears your hippocampus to make room for new information the following day. After being awake for 16 hours, it’s difficult for your hippocampus to hold on to new information. If you’ve stayed up late to read a textbook, and read the same paragraph over and over, failing to comprehend the information, then you’ve experienced a full hippocampus. Light sleep is the delay refresh that renews your ability to learn new facts.

Most of your light sleep is at the end of a full night’s sleep. That means waking up early to study can be counterproductive. When you wake up early and only get five to six hours of sleep, you severely impair your ability to learn. And, if you wake up much earlier than usual (up at 5 a.m. when you typically wake at 7 a.m.), you’re also missing most of your REM sleep that night.

REM Sleep (Dream Sleep)

When you enter REM sleep, your mind begins to make sense of what happened during the day by connecting newly stored information with previously stored information. The connections are often bizarre and lead to creative breakthroughs. Singer/songwriter Paul McCartney famously woke up with the entire melody to ‘Yesterday’ in his head and thought someone else had written it.

REM sleep not only provides creative insights, it offers emotional insights too.

Dreams (which only occur during REM sleep) simulate anxious situations so that you learn how to deal with your anxiety and become coolheaded under pressure. Dreams also help you transition from despair to hope ‐ if you’re going through a bitter break‐up or divorce, dreaming will help you move on. Dreams are the cheapest and most effective form of therapy.

Deep sleep improves your ability to recall information, light sleep improves your ability to learn new information, and REM sleep improves your ability to make sense of information, and any related emotion.

Two Pillars of a Good Night’s Sleep


When the brain detects blue light, it suppresses a chemical called melatonin. That’s not good, because melatonin provides the push you need to fall asleep.

A study found that reading a book on an iPad suppressed melatonin 50% more than reading a print book. Another study found that a bedside lamp, with just one to two percent of the strength of daylight, can also reduce melatonin by 50%. That’s why when the sun goes down, I now put on blue light blocking glasses. Then, a half hour before bed, I read a print book under red LED light. Then, when it’s time to sleep, I put on my sleep mask.


Your body temperature needs to drop two to three degrees Fahrenheit to initiate sleep. It is hard to lower your core temperature when your house remains at room temperature. That’s why I’ve programmed my thermostat to reduce my house temperature to 65F every night at nine p.m., and maintain that temperature throughout the night.

Also, right before bed, I take a hot shower. Taking a hot shower might seem counter‐intuitive…but after a hot shower/bath my body heat transfers from my core to the surface of my body, then dissipates into the atmosphere. The result: core temperature drops, and it’s easy to fall asleep.

For better or for worse, our body clock dictates our sleep patterns.

How do you feel about your alarm clock? Is it the despised, sleep-destroying herald of that hateful time of day, morning? Or, by the time it starts its beeping, are you already out of bed, going energetically about your gung-ho, sunrise-filled life?

To put the question more concisely: Why is morning a balm to some and a bane to others?

Well, it all depends on your built-in body clock.

Nestled deep in the folds of your brain is a primeval timepiece, an internal clock that ticks out your body’s personal circadian rhythm – a 24-hour cycle that, regardless of morning alarms and evening appointments, your body is naturally inclined to follow.

Your circadian rhythm dictates a great number of things. It’s what guides your body’s desire for sleep – or the opposite of sleep. It’s what makes you want food or drink at certain times. It’s even responsible, to an extent, for moodiness and emotional ups and downs, as well as your metabolic rate.

But here’s the thing: circadian rhythms vary from person to person – a fact that causes a large chunk of the population both to hate alarm clocks and to suffer from health complications.

This portion, which makes up about 30 percent of the population, consists of “night owls,” people whose circadian rhythm inclines them to seek slumber late at night and to rise late in the morning.

Distressingly for these nocturnal folks, society is morning-oriented. School and work typically begin in the morning and last through the afternoon, right when the body clocks of night owls say they should be asleep, or at least still sleepy.

Being out of sync with society’s schedule puts night owls in a tough position: they must get up early even though they fall asleep late. Thus, they’re often sleep-deprived, which makes it likelier that they’ll suffer from a range of illnesses, including diabetes, depression and cancer.

Sleep deprivation leads to high blood pressure and heart disease.

Diet, exercise and sleep: the author used to tell people that these three things were the pillars of good health. Now he goes one step further and says that sleep is the foundation on which all the other pillars stand.

This is especially the case for cardiovascular health.

In the West, cardiovascular disease, otherwise known as heart disease, is so prevalent that the cost of treating it is hamstringing the health-care budgets of multiple countries.

But the real cure couldn’t be cheaper. People simply need to sleep more.

Consider a 2011 study which looked at more than 500,000 people from eight different countries – men and women, young and old, and of varying ethnicities – and concluded that sleep deprivation increases a person’s risk of either getting or dying from cardiovascular disease by 45 percent.

Another study traced employed Japanese males over the course of 14 years and found that, when compared to workers who got more sleep, those who slept six hours or less per night were 500 percent more likely to suffer a cardiac arrest.

Even when one accounts for other factors that cause heart problems – such as smoking or obesity – the link between sleep deprivation and cardiovascular disease is strong.

So why does less sleep mean more heart problems? Well, it mainly has to do with blood pressure.

Whenever you don’t get enough sleep, the pressure in your veins goes up. Even losing one or two hours of rest will do the trick. It might take a while, but, eventually, these instances of increased pressure take their toll: the walls of your blood vessels become stretched and damaged.

Now, it’s common knowledge that high blood pressure is responsible for many deaths – seven million of them per year, to be exact – but not many people know that they could avoid a similar fate just by catching a few extra z’s.

Every animal needs sleep, but they don’t all need the same kind of sleep or the same amount of it.

Did you know that sharks never close their eyes? These killers of the deep literally get no shut-eye whatsoever. But that doesn’t mean that, as was long believed, they don’t sleep; they simply have no eyelids.

In fact, we have yet to discover an animal that doesn’t need sleep. And, although the need for sleep is universal, the amount needed varies wildly.

Elephants, for example, require roughly four hours of sleep per day. Big cats, on the other hand, such as lions and tigers, need more than three times that amount – about 15 hours! We humans, as you doubtless know, should sleep for about eight hours per day. And the world’s most accomplished sleeper, the brown bat, clocks so much sleep time that its waking day lasts only five hours.

Why do some animals need so much sleep and others so little? We have no idea.

Species’ snooze habits aren’t governed by size or diet. It doesn’t matter whether they’re nocturnal or diurnal. Even trying to generalize by order doesn’t work. For instance, squirrels and degus both belong to the animal order Rodentia – but degus need 7.7 hours of sleep, and squirrels need more than twice that.

As if these differences weren’t baffling enough, different animal species also engage in different kinds of sleep.

For example, research shows that only two classes of animals experience rapid-eye-movement (REM) sleep: mammals and birds. During the REM stage, the sleeping bird or mammal dreams, and to prevent its acting out those dreams, its entire body becomes paralyzed. But all other animal classes – reptiles, fish, insects and so on – never enter this stage of sleep. In short, your cat probably dreams, but your pet boa constrictor certainly doesn’t.

There’s one exception to this rule: it seems that whales and dolphins and other aquatic mammals don’t enter REM sleep, either. This is probably because were these animals to experience the attendant paralysis, they would drown.

Driving drowsy is equally as dangerous as driving drunk.

People do crazy things for fame, and a long list of these wild feats can be found in The Guinness Book of World Records. However, some categories that the book used to monitor either proved so dangerous or so reprehensible that they’re no longer recorded. One of those categories? Sleep deprivation.

Depriving yourself of sleep is certainly bad for you personally – but it can also make you a threat to society, especially if you regularly operate a motor vehicle.

All it takes is getting less than seven hours of sleep per night, and your chances of becoming involved in an accident increase appreciably.

But that’s not a whole lot of sleep loss, right? True. However, it’s enough to make your body try to catch up on rest by entering a microsleep – a bout of sleep that lasts only a handful of seconds.

A microsleep only differs from normal sleep in duration. So, during those moments of microsleep, you have no control of your motor functions, which means you’re no longer doing what you should do when behind the wheel: attending to the task of driving.

Even if your speed is relatively low – say, 30 miles per hour – it only takes two seconds for your car to drift into the next lane. So a two-second microsleep is more than sufficient to cause a major accident.

But sleeping while driving isn’t the only danger of sleep deprivation. In fact, driving while drowsy is as dangerous as driving while drunk.

Just consider this Australian study. Researchers split participants into two groups. The members of one group were given enough alcohol to make them legally drunk (so, until their blood-alcohol concentration barely exceeded 0.05). The members of the other group were deprived of sleep for a night.

The findings? Well, the sleep-deprived participants, after going without sleep for 19 hours, were given a concentration test – and they did as poorly on it as the drunk group.

Keep this in mind next time you’re out late. You may not have had a drop to drink, but driving drowsy is comparably irresponsible.

Sleepwalking kills – and it’s not caused by dreaming.

One night in 1987, a Canadian man named Kenneth Parks walked to his car, shoeless, and drove 14 miles to the house of his wife’s parents. He then entered the house, took a knife from the kitchen and stabbed his mother-in-law to death. While driving back home, Parks had what must have been a terrible experience: he woke up.

Yes, Parks committed murder while completely asleep, and it’s cases such as his that prompt the author to assert that few other medical fields come with as surprising a range of disorders as sleep studies. It’s a bold claim – but it seems less outrageous when you consider homicidal sleepwalkers such as Parks.

Parks had a history of sleepwalking, so when he woke up in his car with his hands covered in blood, he wasn’t as surprised as a non-sleepwalker might have been. He immediately drove to the local police station and informed the officers on duty that he thought he might have killed someone, though he had no idea who.

Parks was charged with murder, but when the jury found out about his history of sleepwalking, they acquitted him.

Now, we should mention that most sleepwalking is harmless, and that Parks’s case is certainly an outlier. It should also be noted that people probably aren’t dreaming while sleepwalking.

It would seem sensible to assume that, while stabbing his mother-in-law, Parks was dreaming of committing murder. But the evidence doesn’t support this supposition.

In fact, research indicates that people don’t sleepwalk during REM sleep, the stage of sleep during which we dream. If awakened from an episode of sleepwalking, people report an absolute blank – a dreamless nothing.

Interestingly enough, this might explain why sleepwalking is more common among children than adults. Children simply spend less time than adults in the REM stage of sleep, meaning they have more time during the night to sleepwalk.

If you want to sleep better, get lots of sunlight and avoid certain substances.

Most of us don’t get eight hours’ sleep. Or, if we do, the sleep isn’t exactly high-quality. We toss and turn; we wake up in the middle of the night. Our minds are constantly on, thinking about unanswered emails, approaching deadlines and the constant chatter of social media.

Well, the author has some tips that’ll help us all get a nightly eight hours of deep, rejuvenating sleep.

First off, there are a couple of things you might want to consider avoiding – such as alcohol and nicotine.

Sure, a nightcap feels relaxing, and it might help your waking self unwind, but alcohol also makes it harder for your body to enter deep sleep. And large quantities of alcohol can impair your breathing when you’re asleep. Furthermore, people usually wake up when the alcohol wears off, which sort of defeats the purpose of all that pre-sleep relaxation.

Nicotine will also tamper with your slumbers. Smoking may feel as relaxing as drinking, but nicotine is, like alcohol, a stimulant. Thus, smokers tend to sleep lightly – and, because of morning nicotine withdrawal, they often wake up earlier than they’d like.

In addition to avoiding these substances, you can also introduce a few sleep-promoting routines into your day and evening.

For example, before going to bed, take a hot bath. The bath itself will relax your body and mind – and, of equal importance, the drop in body temperature that results from your exiting the bath will induce a feeling of drowsiness.

You should also try to get a good amount of natural sunlight during the day. This will help your body regulate your sleep pattern. Another trick is to open your bedroom curtains before you hop into bed, so that the sun, and not an alarm, is what rouses you in the morning. Finally, keep the temperature in your bedroom relatively low.

With these tips, there’s no reason you can’t start getting the sleep you’ve been dreaming of!


The key message in this book summary:

All animals need sleep. Indeed, it’s the bedrock of good health. With too little sleep, we risk ill health and dangerous lapses of concentration. To live healthy, productive lives, we should listen closely to our body’s internal clock, which tells us when, and how much, to sleep. If you’re having trouble sleeping, avoid alcohol and nicotine, and take a warm bath before bed.

Actionable advice:

If you can’t sleep, get up.

Have you ever lain awake at night, staring at the ceiling, worrying that you’re going to be too tired to function in the morning? If so, then you’re not doing yourself any favors. Ironically, feeling worried about your inability to go to sleep makes it more difficult for you to fall asleep. Therefore, set a time limit on how long you’ll lie in bed awake. After 20 minutes, if you still can’t sleep, get up and engage in some relaxing activity until you feel sleepy again.

About the author

Matthew Walker is a professor of neuroscience and psychology at UC Berkeley, the Director of its Sleep and Neuroimaging Lab, and a former professor of psychiatry at Harvard University. He has published over 100 scientific studies and has appeared on 60 Minutes, Nova, BBC News, and NPR’s Science Friday. Why We Sleep is his first book.


Medicine, Sleep Disorders, Neuroscience, Anatomy, Mental Health, Physical Illness and Disease, Anatomy and Physiology

Table of Contents

Part 1 This Thing Called Sleep
Chapter 1 To Sleep … 3
Chapter 2 Caffeine, Jet Lag, and Melatonin: Losing and Gaining Control of Your Sleep Rhythm 13
Chapter 3 Defining and Generating Sleep: Time Dilation and What We Learned from a Baby in 1952 38
Chapter 4 Ape Beds, Dinosaurs, and Napping with Half a Brain Who Sleeps, How Do We Sleep, and How Much? 56
Chapter 5 Changes in Sleep Across the Life Span 78

Part 2 Why Should You Sleep?
Chapter 6 Your Mother and Shakespeare Knew: The Benefits of Sleep for the Brain 107
Chapter 7 Too Extreme for the Guinness Book of World Records: Sleep Deprivation and the Brain 133
Chapter 8 Cancer, Heart Attacks, and a Shorter Life: Sleep Deprivation and the Body 164

Part 3 How and Why We Dream
Chapter 9 Routinely Psychotic: REM-Sleep Dreaming 193
Chapter 10 Dreaming as Overnight Therapy 206
Chapter 11 Dream Creativity and Dream Control 219

Part 4 From Sleeping Pills to Society Transformed
Chapter 12 Things That Go Bump in the Night: Sleep Disorders and Death Caused by No Sleep 237
Chapter 13 iPads, Factory Whistles, and Nightcaps: What’s Stopping You from Sleeping? 265
Chapter 14 Hurting and Helping Your Sleep: Pills vs. Therapy 282
Chapter 15 Sleep and Society: What Medicine and Education Are Doing Wrong; What Google and NASA Are Doing Right 296
Chapter 16 A New Vision for Sleep in the Twenty-First Century 324

Conclusion: To Sleep or Not to Sleep 340
Appendix: Twelve Tips for Healthy Sleep 341
Illustration Permissions 343
Acknowledgments 344


A New York Times bestseller and international sensation, this “stimulating and important book” (Financial Times) is a fascinating dive into the purpose and power of slumber.

Sleep is one of the most important but least understood aspects of our life, wellness, and longevity. Until very recently, science had no answer to the question of why we sleep, or what good it served, or why we suffer such devastating health consequences when we don’t sleep. Compared to the other basic drives in life—eating, drinking, and reproducing—the purpose of sleep remained elusive.

An explosion of scientific discoveries in the last twenty years has shed new light on this fundamental aspect of our lives. Now, preeminent neuroscientist and sleep expert Matthew Walker gives us a new understanding of the vital importance of sleep and dreaming. Within the brain, sleep enriches 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. Dreaming mollifies painful memories and creates a virtual reality space in which the brain melds past and present knowledge to inspire creativity.

Walker answers important questions about sleep: how do caffeine and alcohol affect sleep? What really happens during REM sleep? Why do our sleep patterns change across a lifetime? How do common sleep aids affect us and can they do long-term damage? Charting cutting-edge scientific breakthroughs, and synthesizing decades of research and clinical practice, Walker explains how we can harness sleep to improve learning, mood, and energy levels; regulate hormones; prevent cancer, Alzheimer’s, and diabetes; slow the effects of aging; increase longevity; enhance the education and lifespan of our children, and boost the efficiency, success, and productivity of our businesses. Clear-eyed, fascinating, and accessible, Why We Sleep is a crucial and illuminating book.


“Why We Sleep is an important and fascinating book…Walker taught me a lot about this basic activity that every person on Earth needs. I suspect his book will do the same for you.” – Bill Gates

“A thoughtful tour through the still dimly understood state of being asleep … Why We Sleep is a book on a mission. Walker is in love with sleep and wants us to fall in love with sleep, too. And it is urgent. He makes the argument, persuasively, that we are in the midst of a ‘silent sleep loss epidemic’ that poses ‘the greatest public health challenge we face in the 21st century’ … Why We Sleep mounts a persuasive, exuberant case for addressing our societal sleep deficit and for the virtues of sleep itself. It is recommended for night-table reading in the most pragmatic sense.” – New York Times Book Review

“The director of UC Berkeley’s Sleep and Neuroimaging Lab explores the purpose of slumber. Understanding the ‘why,’ it turns out, just might help you with the ‘how to.'” – People

“A neuroscientist has found a revolutionary way of being cleverer, more attractive, slimmer, happier, healthier and of warding off cancer — a good night’s shut-eye … It’s probably a little too soon to tell you that Why We Sleep saved my life, but I can tell you that it’s been an eye-opener.” – The Guardian

“This is a stimulating and important book which you should read in the knowledge that the author is, as he puts it, ‘in love with everything that sleep is and does.’ But please do not begin it just before bedtime.” – Financial Times

“Fascinating … Walker describes how our resting habits have changed throughout history; the connection between sleep, chronic disease, and life span; and why the pills and aids we use to sleep longer and deeper are actually making our nights worse. Most important, he gives us simple, actionable ways to get better rest—tonight.” – Men’s Journal

“Walker is a scientist but writes for the layperson, illustrating tricky concepts with easily grasped analogies. Of particular interest to business owners, educators, parents, and government officials, and anyone who has ever suffered from a poor night’s sleep.” – Library Journal, starred review

“Why We Sleep is simply a must-read. World-renowned neuroscientist and sleep expert Matthew Walker takes us on a fascinating and indispensable journey into the latest understandings of the science of sleep. And the book goes way beyond satisfying intellectual curiosity, as it explores the cognitive, health, safety and business consequences of compromising the quality and quantity of our sleep; insights that may change the way you live your life. In these super-charged, distracting times it is hard to think of a book that is more important to read than this one.” – Adam Gazzaley, co-author of The Distracted Mind, founder and executive director of Neuroscape, and Professor of Neurology, Physiology, and Psychiatry at University of California, San Francisco

“Most of us have no idea what we do with a third of our lives. In this lucid and engaging book, Matt Walker explains the new science that is rapidly solving this age-old mystery. Why We Sleep is a canny pleasure that will have you turning pages well past your bedtime.” – Daniel Gilbert, professor of psychology at Harvard and author of Stumbling on Happiness

“In Why We Sleep, Dr. Matt Walker brilliantly illuminates the night, explaining how sleep can make us healthier, safer, smarter, and more productive. Clearly and definitively, he provides knowledge and strategies to overcome the life-threatening risks associated with our sleep-deprived society. Our universal need for sleep ensures that every reader will find value in Dr. Walker’s insightful counsel.” – Mark R. Rosekind, Ph.D., former NHTSA Administrator, NTSB member, and NASA scientist

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CHAPTER 1 To Sleep…
Do you think you got enough sleep this past week? Can you recall the last time you woke up without an alarm clock feeling refreshed, not needing caffeine? If the answer to either of these questions is “no,” you are not alone. More than a third of adults in many developed nations fail to obtain the recommended seven to nine hours of nightly sleep.I

I doubt you are surprised by this fact, but you may be surprised by the consequences. Routinely sleeping less than six hours a night weakens your immune system, substantially increasing your risk of certain forms of cancer. Insufficient sleep appears to be a key lifestyle factor linked to your risk of developing Alzheimer’s disease. Inadequate sleep—even moderate reductions for just one week—disrupts blood sugar levels so profoundly that you would be classified as pre-diabetic. Short sleeping increases the likelihood of your coronary arteries becoming blocked and brittle, setting you on a path toward cardiovascular disease, stroke, and congestive heart failure. Fitting Charlotte Brontë’s prophetic wisdom that “a ruffled mind makes a restless pillow,” sleep disruption further contributes to all major psychiatric conditions, including depression, anxiety, and suicidality.

Perhaps you have also noticed a desire to eat more when you’re tired? This is no coincidence. Too little sleep swells concentrations of a hormone that makes you feel hungry while suppressing a companion hormone that otherwise signals food satisfaction. Despite being full, you still want to eat more. It’s a proven recipe for weight gain in sleep-deficient adults and children alike. Worse, should you attempt to diet but don’t get enough sleep while doing so, it is futile, since most of the weight you lose will come from lean body mass, not fat.

Add the above health consequences up, and a proven link becomes easier to accept: relative to the recommended seven to nine hours, the shorter your sleep, the shorter your life span. The old maxim “I’ll sleep when I’m dead” is therefore unfortunate. Adopt this mind-set, and it is possible that you will be dead sooner and the quality of that (shorter) life will be worse. The elastic band of sleep deprivation can stretch only so far before it snaps. Sadly, human beings are in fact the only species that will deliberately deprive themselves of sleep without legitimate gain. Numerous components of wellness, and countless seams of societal fabric, are being eroded by our costly state of sleep neglect: human and financial alike. So much so that the Centers for Disease Control declared insufficient sleep as a public health epidemic. It may not be a coincidence that countries where sleep time has declined most dramatically over the past century, such as the US, the UK, Japan, and South Korea, and several in western Europe, are also those suffering the greatest increase in rates of the aforementioned physical diseases and mental disorders.

Scientists such as myself have even started lobbying doctors to start “prescribing” sleep. As medical advice goes, it’s perhaps the most painless and enjoyable to follow. Do not, however, mistake this as a plea to doctors to start prescribing more sleeping pills—quite the opposite, in fact, considering the evidence surrounding the deleterious health consequences of these drugs.

But can we go so far as to say that a lack of sleep can kill you outright? Quite posssibly—on at least two counts. First, there is a very rare genetic disorder that starts with a progressive insomnia, emerging in midlife. Several months into the disease course, the patient stops sleeping altogether. By this stage, they have started to lose many basic brain and body functions. Few drugs that we currently have will help the patient sleep. After twelve to eighteen months of no sleep, the patient will die.

Second is the deadly circumstance of getting behind the wheel of a motor vehicle without having had sufficient sleep. Drowsy driving is the cause of hundreds of thousands of traffic accidents and fatalities each year. And here, it is not only the life of the sleep-deprived individuals that is at risk, but the lives of those around them. Tragically, one person dies in a traffic accident every hour in the United States due to a fatigue-related error.

Society’s apathy toward sleep has, in part, been caused by the historic failure of science to explain why we need it. Sleep remained one of the last great biological mysteries. All of the mighty problem-solving methods in science—genetics, molecular biology, and high-powered digital technology—have been unable to unlock the stubborn vault of sleep. Minds of the most stringent kind, including Nobel Prize–winner Francis Crick, who deduced the twisted-ladder structure of DNA, famed Roman educator and rhetorician Quintilian, and even Sigmund Freud had all tried their hand at deciphering sleep’s enigmatic code, all in vain.

To better frame this state of prior scientific ignorance, imagine the birth of your first child. At the hospital, the doctor enters the room and says, “Congratulations, it’s a healthy baby boy. We’ve completed all of the preliminary tests and everything looks good.” She smiles reassuringly and starts walking toward the door. However, before exiting the room she turns around and says, “There is just one thing. From this moment forth, and for the rest of your child’s entire life, he will repeatedly and routinely lapse into a state of apparent coma. It might even resemble death at times. And while his body lies still his mind will often be filled with stunning, bizarre hallucinations. This state will consume one-third of his life and I have absolutely no idea why he’ll do it, or what it is for. Good luck!”

Astonishing, but until very recently, this was reality: doctors and scientists could not give you a consistent or complete answer as to why we sleep. Consider that we have known the functions of the three other basic drives in life—to eat, to drink, and to reproduce—for many tens if not hundreds of years now. Yet the fourth main biological drive, common across the animal kingdom—the drive to sleep—has continued to elude science for millennia.

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.”II

Yet sleep has persisted. Heroically so. Indeed, every animal species carefully studied to date sleeps.III This suggests that sleep evolved with—or very soon after—life itself on our planet. Moreover, the subsequent perseverance of sleep throughout evolution means there must be tremendous benefits that far outweigh all of the obvious hazards and detriments.

Ultimately, asking “Why do we sleep?” was the wrong question. It implied there was a single function, one holy grail of a reason that we slept, and we went in search of it. Theories ranged from the logical (a time for conserving energy), to the peculiar (an opportunity for eyeball oxygenation), to the psychoanalytic (a non-conscious state in which we fulfill repressed wishes).

This book will reveal a very different truth: sleep is infinitely more complex, profoundly more interesting, and strikingly health-relevant. We sleep for a rich litany of functions, plural—an abundant constellation of nighttime benefits that service both our brains and our bodies. There does not seem to be one major organ within the body, or process within the brain, that isn’t optimally enhanced by sleep (and detrimentally impaired when we don’t get enough). That we receive such a bounty of health benefits each night should not be surprising. After all, we are awake for two-thirds of our lives, and we don’t just achieve one useful thing during that stretch of time. We accomplish myriad undertakings that promote our own well-being and survival. Why, then, would we expect sleep—and the twenty-five to thirty years, on average, it takes from our lives—to offer one function only?

Through an explosion of discoveries over the past twenty years, we have come to realize that evolution did not make a spectacular blunder in conceiving of sleep. Sleep dispenses a multitude of health-ensuring benefits, yours to pick up in repeat prescription every twenty-four hours, should you choose.

Within the brain, sleep enriches a diversity of functions, including our ability to learn, memorize, and make logical decisions and choices. Benevolently servicing our psychological health, sleep recalibrates our emotional brain circuits, allowing us to navigate next-day social and psychological challenges with cool-headed composure. We are even beginning to understand the most impervious and controversial of all conscious experiences: the dream. Dreaming provides a unique suite of benefits to all species fortunate enough to experience it, humans included. Among these gifts are a consoling neurochemical bath that mollifies painful memories and a virtual reality space in which the brain melds past and present knowledge, inspiring creativity.

Downstairs in the body, sleep restocks the armory of our immune system, preventing infection and warding off all manner of sickness. Sleep reforms the body’s metabolic state by fine-tuning the balance of insulin and circulating glucose. Sleep further regulates our appetite, helping control body weight through healthy food selection rather than rash impulsivity. Plentiful sleep maintains a flourishing microbiome within your gut from which we know so much of our nutritional health begins. Adequate sleep is intimately tied to the fitness of our cardiovascular system, lowering blood pressure while helping keep our hearts in fine condition.

A balanced diet and exercise are of vital importance, yes. But we now see sleep as a preeminent force in this health trinity. The physical and mental impairments caused by one night of bad sleep dwarf those caused by an equivalent absence of food or exercise. It is difficult to imagine any other state—natural or medically manipulated—that affords a more powerful redressing of physical and mental health at every level of analysis.

Based on a rich, new scientific understanding of sleep, we no longer have to ask what sleep is good for. Instead, we are now forced to wonder whether there are any biological functions that do not benefit by a good night’s sleep.

Emerging from this research renaissance is an unequivocal message: 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. Unfortunately, the real evidence that makes clear all of the dangers that befall individuals and societies when sleep becomes short have not been clearly telegraphed to the public. It is perhaps the most glaring omission in the contemporary health conversation. In response, this book is intended to help address this unmet need, and provide what I hope is a fascinating journey of discoveries. It aims to revise our cultural appreciation of sleep, and reverse our neglect of it.

Personally, I should note that I am rather in love with sleep (not just my own, though I do give myself a non-negotiable eight-hour sleep opportunity each night). I am in love with everything sleep is and does. I am in love with discovering all that remains unknown about it. I am in love with communicating the relevance of it to the public. I am in love with finding any and all methods for reuniting humanity with the sleep it so desperately needs. This love affair has now spanned a twenty-plus-year research career that began when I was a professor of psychiatry at Harvard Medical School and continues now that I am a professor of neuroscience and psychology at the University of California, Berkeley.

It was not, however, love at first sight. I am an accidental sleep researcher. It was never my intent to inhabit this esoteric outer territory of science. At age eighteen I went to study at the Queen’s Medical Center in England: a prodigious institute in Nottingham boasting a wonderful band of brain scientists on its faculty. Ultimately, medicine wasn’t for me, as it seemed more concerned with answers, whereas I was always more enthralled by questions. For me, answers were simply a way to get to the next question. I decided to study neuroscience, and after graduating, obtained my PhD in neurophysiology supported by a fellowship from England’s Medical Research Council, London.

It was during my PhD work, conducted mostly at Newcastle University, that I began making my first scientific contributions in the field of sleep research. I was examining patterns of electrical brainwave activity in older adults in the early stages of dementia. Counter to common belief, there isn’t just one type of dementia. Alzheimer’s disease is the most common, but is only one of many types. For a number of treatment reasons, it is critical to know which type of dementia an individual is suffering from as soon as possible.

I began assessing brainwave activity from my patients during wake and sleep. My hypothesis: there was a unique and specific electrical brain signature that could forecast which dementia subtype each individual was progressing toward. Measurements taken during the day were ambiguous, with no clear signature of difference to be found. Only in the nighttime ocean of sleeping brainwaves did the recordings speak out a clear labeling of my patients’ saddening disease fate. The discovery proved that sleep could potentially be used as a new early diagnostic litmus test to understand which type of dementia an individual would develop.

Sleep became my obsession. The answer it had provided me, like all good answers, only led to more fascinating questions, among them: Was the disruption of sleep in my patients actually contributing to the diseases they were suffering from, and even causing some of their terrible symptoms, such as memory loss, aggression, hallucinations, delusions? I read all I could. A scarcely believable truth began to emerge—nobody actually knew the clear reason why we needed sleep, and what it does. I could not answer my own question about dementia if this fundamental first question remained unanswered. I decided I would try to crack the code of sleep.

I halted my research in dementia and, for a post-doctoral position that took me across the Atlantic Ocean to Harvard, set about addressing one of the most enigmatic puzzles of humanity—one that had eluded some of the best scientists in history: Why do we sleep? With genuine naïveté, not hubris, I believed I would find the answer within two years. That was twenty years ago. Hard problems care little about what motivates their interrogators; they meter out their lessons of difficulty all the same.

Now, after two decades of my own research efforts, combined with thousands of studies from other laboratories around the world, we have many of the answers. These discoveries have taken me on wonderful, privileged, and unexpected journeys inside and outside of academia—from being a sleep consultant for the NBA, NFL, and British Premier League football teams; to Pixar Animation, government agencies, and well-known technology and financial companies. These sleep revelations, together with many similar discoveries from my fellow sleep scientists, will offer proof about the vital importance of sleep.

A final comment on the structure of this book. The chapters are written in a logical order, traversing a narrative arc in four main parts.

Part 1 demystifies this beguiling thing called sleep: what it is, what it isn’t, who sleeps, how much they sleep, how human beings should sleep (but are not), and how sleep changes across your life span or that of your child, for better and for worse.

Part 2 details the good, the bad, and the deathly of sleep and sleep loss. We will explore all of the astonishing benefits of sleep for brain and for body, affirming what a remarkable Swiss Army knife of health and wellness sleep truly is. Then we turn to how and why a lack of sufficient sleep leads to a quagmire of ill health, disease, and untimely death—a wakeup call to sleep if ever there was one.

Part 3 offers safe passage from sleep to the fantastical world of dreams scientifically explained. From peering into the brains of dreaming individuals, and precisely how dreams inspire Nobel Prize–winning ideas that transform the world, to whether or not dream control really is possible, and if such a thing is even wise—all will be revealed.

Part 4 seats us first at the bedside, explaining numerous sleep disorders, including insomnia. I will unpack the obvious and not-so-obvious reasons for why so many of us find it difficult to get a good night’s sleep, night after night. A discussion of sleeping pills then follows, based on scientific and clinical data. Details of new, safer, and more effective non-drug therapies for better sleep will then be described. Transitioning from bedside up to the level of sleep in society, we will subsequently learn of the sobering impact that insufficient sleep has in education, in medicine and health care, and in business. The evidence shatters beliefs about the usefulness of long waking hours with little sleep in effectively, safely, profitably, and ethically accomplishing the goals of each of these disciplines. Concluding the book with genuine optimistic hope, I lay out a road map of ideas that can reconnect humanity with the sleep it remains so bereft of—a new vision for sleep in the twenty-first century.

I should note that you need not read this book in this progressive, four-part narrative arc. Each chapter can, for the most part, be read individually, and out of order, without losing too much of its significance. I therefore invite you to consume the book in whole or in part, buffet-style or in order, all according to your personal taste.

It is worthwhile pointing out that this book is not designed to be a self-help guide. It is not written to target or treat sleep disorders, including insomnia. There are books that do this, and many of them will recommend speaking to a doctor if you suspect you have a sleep disorder. I am also very understanding of, and sympathetic to, those people who struggle with sleep and are most anxious about it. For these individuals, it is possible that their anxiety may increase when reading about the impact of insufficient sleep, including information contained in the book. I therefore want to alert the reader to this possibility, allowing for reader discretion on this matter.

In closing, I offer a disclaimer. Should you feel drowsy and fall asleep while reading the book, unlike most authors, I will not be disheartened. Indeed, based on the topic and content of this book, I am actively going to encourage that kind of behavior from you. Knowing what I know about the relationship between sleep and memory, it is the greatest form of flattery for me to know that you, the reader, cannot resist the urge to strengthen and thus remember what I am telling you by falling asleep. So please, feel free to ebb and flow into and out of consciousness during this entire book. I will take absolutely no offense. On the contrary, I would be delighted.

I. The Centers for Disease Control (CDC) stipulates that adults need seven hours of sleep or more per twenty-four hours.

II. Dr. Allan Rechtschaffen.

III. Cirelli, C., and Tononi, G. (2008). “Is sleep essential?” PLoS Biology. 6, e216.

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