Why We Sleep Read online

  Matthew Walker

  * * *

  WHY WE SLEEP

  The New Science of Sleep and Dreams

  Contents

  – Part 1 –

  This Thing Called Sleep

  Chapter 1 To Sleep …

  Chapter 2 Caffeine, Jet Lag, and Melatonin: Losing and Gaining Control of Your Sleep Rhythm

  Chapter 3 Defining and Generating Sleep: Time Dilation and What We Learned from a Baby in 1952

  Chapter 4 Ape Beds, Dinosaurs, and Napping with Half a Brain: Who Sleeps, How Do We Sleep, and How Much?

  Chapter 5 Changes in Sleep Across the Life Span

  – Part 2 –

  Why Should You Sleep?

  Chapter 6 Your Mother and Shakespeare Knew: The Benefits of Sleep for the Brain

  Chapter 7 Too Extreme for the Guinness Book of World Records: Sleep Deprivation and the Brain

  Chapter 8 Cancer, Heart Attacks, and a Shorter Life: Sleep Deprivation and the Body

  – Part 3 –

  How and Why We Dream

  Chapter 9 Routinely Psychotic: REM-Sleep Dreaming

  Chapter 10 Dreaming as Overnight Therapy

  Chapter 11 Dream Creativity and Dream Control

  – 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

  Chapter 13 iPads, Factory Whistles, and Nightcaps: What’s Stopping You from Sleeping?

  Chapter 14 Hurting and Helping Your Sleep: Pills vs. Therapy

  Chapter 15 Sleep and Society: What Medicine and Education Are Doing Wrong; What Google and NASA Are Doing Right

  Chapter 16 A New Vision for Sleep in the Twenty-First Century

  Conclusion: To Sleep or Not to Sleep

  Appendix: Twelve Tips for Healthy Sleep

  Illustration Permissions

  Acknowledgments

  Follow Penguin

  Matthew Walker’s fascination with sleep has taken him from Nottingham to Harvard and on to the University of California, Berkeley, where he is currently Professor of Neuroscience and Psychology and Director of the Sleep and Neuroimaging Laboratory. He has published over 100 scientific research studies during the course of his twenty-year career. Why We Sleep is his first book.

  PENGUIN BOOKS

  WHY WE SLEEP

  ‘This is brilliant popular science writing: accessible, compelling and enlightening.’ David Lodge, The Times Literary Supplement, Books of the Year

  ‘One of my favourite books this year, and definitely the most useful. Now I go to sleep much earlier and feel much better for it.’ William Leith, Evening Standard, Books of the Year

  ‘A stimulating and important book which you should read … indisputable … richly packed with science … Walker describes superbly what is going on in the brain during sleep and dreaming’ Clive Cookson, Financial Times

  ‘Draws on current research to demystify sleep, traverse the wild world of dreams and disentangle sleep disorders. From an infant’s polyphasic snippets of slumber to the “hyper-associative problem-solving benefits” of REM dreaming, Walker’s investigation is anything but soporific’ Barbara Kiser, Nature

  ‘I urge you all to read this book’ Richard Joyner, Times Higher Education

  ‘Good company: congenial, often funny … Why We Sleep is a book on a mission … One especially winning attribute of Walker is that he’s not a scold. He frames his suggestions for more healthful sleep habits not as a series of eat-your-Wheaties admonitions, but as wondrous, uplifting improvements in quality of life … Why We Sleep mounts a persuasive, exuberant case for addressing our societal sleep deficit and for the virtues of sleep itself’ David Kamp, The New York Times Book Review

  ‘A top sleep scientist argues that sleep is more important for our health than diet or exercise’ The Times

  ‘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, author of Stumbling on Happiness

  ‘The REAL scientific facts about sleep’ Daily Mail

  ‘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 … 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

  ‘A bursting rocket of a book … lands shock statistics like a boxer demolishing a tottering opponent … We tend to imagine sleep as if it were not really part of who we are. Walker shows, triumphantly, how dangerously wrong we can be’ James McConnachie, Sunday Times

  ‘Things have to change in our workplace and our communities, our homes and our families … The evidence is enough to send anyone early to bed. It’s no kind of choice at all’ Rachel Cooke, Observer

  ‘Startling, sobering, vital, a life raft … A neuroscientist shows how a good night’s shut-eye can make us cleverer, more attractive, slimmer, happier, healthier and ward off cancer … filled with startling information 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’ Mark O’Connell, Guardian

  ‘He is an expert, but more importantly, he knows how to explain it all clearly to general readers … a well-organized, highly accessible, up-to-date report on sleep and its crucial role in a healthy life’ Kirkus Review

  To Dacher Keltner, for inspiring me to write.

  Part 1

  * * *

  THIS THING CALLED SLEEP

  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. Two-thirds of adults throughout all developed nations fail to obtain the recommended eight hours of nightly sleep.fn1

  I doubt you are surprised by this fact, but you may be surprised by the consequences. Routinely sleeping less than six or seven hours a night demolishes your immune system, more than doubling your risk of cancer. Insufficient sleep is a key lifestyle factor determining whether or not you will develop 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: 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 you will be dead sooner and the quality of that (shorter) li
fe 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. Every component 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 World Health Organization (WHO) has now declared a sleep loss epidemic throughout industrialized nations.fn2 It is no 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 alarming 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? Actually, yes—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. No drugs that we currently have will help the patient sleep. After twelve to eighteen months of no sleep, the patient will die. Though exceedingly rare, this disorder asserts that a lack of sleep can kill a human being.

  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. It is disquieting to learn that vehicular accidents caused by drowsy driving exceed those caused by alcohol and drugs combined.

  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 entire 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.”fn3

  Yet sleep has persisted. Heroically so. Indeed, every species studied to date sleeps.fn4 This simple fact establishes 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 alarmingly more 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. (Many don’t.)

  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, helping fight malignancy, 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 keeping our hearts in fine condition.

  A balanced diet and exercise are of vital importance, yes. But we now see sleep as the 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. So far, the results of thousands of studies insist that no, there aren’t.

  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 the most glaring omission in the contemporary health conversation. In response, this book is intended to serve as a scientifically accurate intervention addressing this unmet need, and 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 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 astonishing brilliance 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.