W H Y W E S L E E P 5 7 mals sleep: 仕 om shrews to parrots, kangaroos, polar bears, bats, and, 0f course, we humans. Sleep iS universal. Even invertebrates, such as primordial mollusks and echinoderms, and even very primitive worms, enJOY periods Of slumber. ⅲ these phases, affectionately termed "lethargus," they, like humans, become unresponsive tO external stimuli. And just as we fall asleep faster and sleep more soundly when sleep-deprived, so, t00 , do worms, defined by their degree ofinsensitivity tO prods 仕 om experimenters. How "old" does this make sleep? Worms emerged during the Cam- brian explosion: at least 500 million years ago. That is, worms (and sleep by association) predate vertebrate life. This includes dinosaurs, which, by inference, are likely to have slept. lmagine diplodocuses and triceratopses comfortably settling in for a night 0f full repose! Regress evolutionary time still further and we have discovered that the very simplest forms of unicellular organisms that survive for pen- Ods exceeding twenty-four hours, such as bacteria, have active and pas- sive phases that correspond t0 the light-dark cycle 0f our planet. lt is a pattern that we now believe tO be the precursor Of our own circadian rhythm, and with it, wake and sleep. Many ofthe explanations for why we sleep circle around a common, and perhaps erroneous, idea: sleep is the state we must enter in order tO fix that which has been upset by wake. But what ifwe turned this argu- ment on its head? What if sleep is so useful—so physiologically ben- eficial to every aspect of our being—that the real question is:Why did life ever bother to wake up? Considering how biologically damaging the state Of wakefulness can Often be, that is the true evolutionary puzzle here, not sleep. Adopt this perspective, and we can pose a very different theory: sleep was the first state 0f life on this planet, and it was 仕 om sleep that wakefulness emerged. lt may be a preposterous hypothesis, and one that nobody is taking seriously or exploring, but personally I do not think it tO be entirely unreasonable. Whichever ofthese MO theories is true, what we know for certain is that sleep is of ancient 0 ⅱ n. lt appeared with the very earliest forms of planetary life. Like other rudimentary features, such as DNA, sleep has remained a common bond uniting every creature in the animal kingdom. A long-lasting commonality, yes; however, there are truly

Conclusion e 孕 or ル 0 0 e 孕 Within the space of a mere hundred years, human beings have aban- doned their bi010 c y mandated need for adequate sleep—one that evolution spent 3 , 400 , 000 years perfecting in service of life-support functions. AS a result, the decimation ofsleep throughout industrialized nations is having a catastrophic impact on our health, our life expec- tancy, our safety, our productivity, and the education 0f our children. This silent sleep loss epidemic is the greatest public health challenge we face ⅲ the fiventy-first century in developed nations. If we wish tO avoid the suffocating noose of sleep neglect, the premature death it inflicts, and the sickening health it invites, a radical shift in our per- sonal, cultural, professional, and societal appreciation of sleep must occur. I believe it is time for us to reclaim our right to a んⅡ night of sleep, without embarrassment or the damaging stigma of laziness. ln doing SO, we can be reunited with that most powerful elixir Of wellness and ⅵ t 曲 dispensed through every conceivable biological pathway. Then we may remember what it feels like t0 be truly awake during the day, infused with the very deepest plenitude ofbeing.

6 MAITHEW WALKER entists could not glve you a consistent or complete answer as tO why we sleep. Consider that we have known the functions Ofthe three Other basic drives ⅲ life—to eat, tO drink, and tO reproduce—for many tens ifnot hundreds ofyears now. Yet the fourth main biological drive, com- mon across the entire animal kingdom—the drive tO sleep—has contin- ued tO elude science for millennia, Addressing the question ofwhy we sleep 仕 om an evolutionary per- spective only compounds the mystery. NO matter what vantage point you take, sleep would appear t0 be the most f001ish ofbiological phe- nomena. 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 t0 predation. Sleep is surely one 0f the most puzzling 0f all human behaviors. On any one 0f these grounds—never mind 0f 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, "lf sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made. " Yet sleep has persisted. Heroically so. lndeed, every species studied t0 date sleeps.t 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 tre- mendous benefits that far outweigh all 0f the obvious hazards and detriments. Ultimately, asking Why d0 we sleep?" was the wrong question. lt implied there was a single function, one h01y 1 0f a reason that we slept, and we went in search 0f it. Theories ranged from the logi- cal (a time for conserving energy), to the peculiar ()n 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 *Dr. AIlan Rechtschaffen. tKushida, C. E 〃 c / e 2 & e 孕 , Volume 1 (Elsever, 2013 ).

4 4 MATTHEW WALKER Had I not added the vertical dashed lines demarcating each ninety- minute cycle, you may have protested that you could not see a regularly repeating ninety-minute pattern. At least not the one you were expect- ing 仕 om my description above. The cause is another peculiar feature of sleep: a lopsided profile 0f sleep stages. While it is true that we flip-flop back and forth between NREM and REM sleep throughout the night every ninety minutes, the ratio 0fNREM sleep to REM sleep within each ninety-minute cycle changes dramatically across the night. ln the first half of the night, the vast majority of our ninety-minute cycles are con- sumed by deep NREM sleep, and very little REM sleep, as can be seen in cycle 1 Ofthe figure above. But as we transition through intO the sec- ond half of the night, this seesaw balance shifts, with most of the time dominated by REM sleep, with little, if any, deep NREM sleep. CycIe 5 is a perfect example ofthis REM-rich 与甲 e of sleep. Why did Mother Nature design this strange, complex equation of unfolding sleep stages? Why cycle between NREM and REM sleep over and over? Why not obtain all ofthe required NREM sleep first, followed by 0f the necessary REM sleep second? Or vice versa? If that's t00 much a gamble on the off chance that an animal only obtains a partial night 0f sleep at some point, then why not keep the ratio within each cycle the same, placing similar proportions ofeggs in both baskets, as it were, rather than putting most ofthem ⅲ one early on, and then invert- ing that imbalance later in the night? Why vary it? lt sounds like an exhausting amount of evolutionary hard work to have designed such a convoluted system, and put it intO biological action. We have no scientific consensus as to why our sleep (and that of 引 1 0ther mammals and birds) cycles in this repeatable but dramatically asymmetric pattern, though a number Of theories exist. One theory I have offered is that the uneven back-and-forth interplaybetween NREM and REM sleep is necessary to elegantly remodel and update our neural circuits at night, and in dOing SO manage the finite storage space within the brain. Forced by the known storage capacity imposed by a set num- ber Of neurons and connections within their memory structures, our brains must find the "sweet spot" between retention of old information and leaving sufficient room for the new. Balancing this storage equation requires identifying which memories are fresh and salient, and which

MATHEW WALKER meaning around," and 市〃 , derivative Of 市 a 襯 , meaning day"). lndeed, every living creature on the planet with a life span 0f more than several days generates this natural cycle. The internal twenty- four-hour clock within your brain communicates its daily circadian rhythm signal t0 every other regon of your brain and every organ in your body. Your twenty-four-hour tempo helps to determine when you want tO be awake and when you want tO be asleep. But it controls Other rhythmic patterns, t00. These include your timed preferences for eat- ing and drinking, your moods and emotions, the amount Of urine you produce,* your core bOdy temperature, your metabolic rate, and the release Of numerous hormones. lt is no coincidence that the likelihood ofbreaking an Olympic record has been clearly tied to time ofday, being maximal at the natural peak ofthe human circadian rhythm in the early afternoon. Even the timing ofbirths and deaths demonstrates circadian rhythmicity due t0 the marked swings in key life-dependent metabolic, cardiovascular, temperature, and hormonal processes that this pace- maker controls. Long before we discovered this biological pacemaker, an ingenious experiment did something utterly remarkable: stopped time—at least, for a plant. lt was in 1729 when French geophysicist Jean-Jacques d'Ortous de Mairan discovered the very first evidence that plants gen- erate their own internal time. De Mairan was studying the leaf movements 0f a species that dis- played heliotropism: when a plant's leaves or flowers track the trajec- tory Ofthe sun as it moves across the sky during the day.. De Mairan was intrigued by one plant ⅲ particular, called M / 襯 0 2 ″市 ca.t N0t only do the leaves ofthis plant trace the arching daytime passage ofthe sun across the sky's face, but at night, they collapse down, almost as though they had wilted. Then, at the start of the following day, the leaves pop *I should note, 仕 om personal experience, that this is a winning fact tO dispense at dinner parties, family gatherings, or Other such social occasions. lt will almost guarantee nobody will approach or speak to you again for the rest 0f the evening, and you'll so never be invited back. tThe word レ市 ca is 仕 om the Latin meaning h ゞ or "bashful," since the leaves will so coll 叩 se down ifyou touch or stroke them.

WHY WE SLEEP 2 4 ー deprived individuals can sleep, if only they would take the appropriate time tO dO SO. lnsomnia is the opposite: (i) suffering 仕 om an 加 adeq リ 4 わ t0 generate sleep, despite ( ⅱ ) allowing oneself the 9 リ砒 e 0 臾 20r ル〃 / t0 get sleep. People suffering 仕 om insomnia therefore can- not produce sufficient sleep quantity/quality, even though they give themselves enough time tO dO SO (seven tO nine hours). Before moving on, it is worth noting the condition Of sleep-state misperception, SO known as paradoxical insomnia. Here, patients will report having slept poorly throughout the night, or even not sleep- ing at . However, when these individuals have their sleep monitored objectively using electrodes or Other accurate sleep monitoring devices, there is a mismatch. The sleep recordings indicate that the patient has slept far better than they themselves believe, and sometimes indicate that a completely んⅡ and healthy night 0f sleep occurred. Patients suf- fering 仕 OI paradoxical insomnia therefore have an illusion, or misper- ception, Ofpoor sleep that is not actually poor. As a result, such patients are treated as hypochondriacal. Though the term may seem dismissive or condescending, it is taken very seriously by sleep medicine doctors, and there are psychological interventions that help after the diagnosis iS made. Returning tO the condition Oftrue insomnia, there are several differ- ent sub-types, ⅲ the same way that there are numerous different forms Of cancer, for example. One distinction separates insomnia intO VO kinds. The first is sleep 0 insomnia, which is difficulty falling asleep. The second is sleep 襯厩 e 〃〃 ce insomnia, or difficulty staying asleep. As the actor and comedian Billy Crystal has said when describing his own battles with insomnia, "I sleep like a baby—l wake up every hour." Sleep onset and sleep maintenance insomnia are not mutually exclu- sive: you can have one or the Other, or bOth. NO matter which Of these kinds of sleep problems is occurring, sleep medicine has very specific clinical boxes that must be checked for a patient tO receive a diagnosis ofinsomnia. For now, these are: ◆ Dissatisfaction with sleep quantity or quality (). g. , difficulty fall- ing asleep, staying sleep, early-morning awakening) ◆ Suffering significant distress or daytime impairment

7 0 MATTHEW WALKER preordained arrangement ofbiphasic sleep—one that our genetic code nevertheless tries tO rekindle every afternoon. The separation 仕 om biphasic sleep occurred at, or even before, our shift 伝 om an agrarian existence tO an industrial one. Anthropological studies ofpre-industrial hunter-gatherers have also dispelled a popular myth about how humans should sleep. * Around the close Of the early modern era (circa late seventeenth and early eighteenth centuries), historical texts suggest that Western Europeans would take vo long bouts of sleep at night, separated by several hours of wakefulness. Nestled in-between these twin slabs of sleep—some- times called first sleep and second sleep, they would read, write, pray, make love, and even socialize. This practice may very well have occurred during this moment in human history, in this geographical region. Yet the fact that no pre- industrial cultures studied tO date demonstrate a similar nightly split-shift of sleep suggests that it is not the natural, evolutionarily programmed form 0f human sleep. Rather, it appears t0 have been a cultural phenomenon that appeared and was popularized with the western European migration. Furthermore, there is no biological rhythm—of brain activity, neurochemical activity, or metabolic activ- ity—that would hint at a human desire to wake up for several hours in the middle 0f the night. lnstead, the true pattern of biphasic sleep— for which there is anthropologlcal, biological, and genetic evidence, and WhiCh remams measurable in human beings tO date—is one consisting 0f a longer bout of continuous sleep at night, followed by a shorter midafternoon nap. Accepting that this is our natural pattern of slumber, can we ever know for certain what types of health consequences have been caused by our abandonment of biphasic sleep? Biphasic sleep is still observed in several siesta cultures throughout the world, including reglons of South America and Mediterranean Europe. When I was a child in the 1980S , I went on vacation to Greece with my family.. As we walked the streets Of the maJOr metropolitan Greek cities we visited, there were signs hanging in storefront windows that were very different 仕 om those 物 A. Roger Ekirch,AtDay3 Close: Night 加襯 e & 22 立 (NewYork: W. W. Norton, 2 開 6 ).

WHY WE SLEEP 2 4 7 something that would feel bad, or trying to accomplish something that would feel good. This law of 叩 proach and avoidance dictates most of human and animal behavior 仕 om a very early age. The forces that implement this law are positive and negative emo- tions. Emotions make us dO things, as the name suggests (remove the first letter 仕 om the word). They motivate our remarkable achievements, incite us t0 try again when we fail, keep us safe 仕 om potential harm, urge us t0 accomplish rewarding and beneficial outcomes, and compel us tO cultivate social and romantic relationships. ln short, emotions in appropriate amounts make life worth living. They offer a healthy and vital existence, psychologically and biologically speaking. Take them away, and you face a sterile existence with no highs or lows t0 speak 矼 Emotionless, you will simply exist, rather than 1 ⅳ e. TragicaIIy, this is the very kind of reality many narcoleptic patients are forced to adopt for reasons we Will now explore. Medically, narcolepsy is considered to be a neurological disorder, meamng that its origns are within the central nervous system, specifi- cally the brain. The condition usually emerges between ages ten and twenty years. There is some genetic basis tO narcolepsy, but it is not inherited. lnstead, the genetic cause appears to be a mutation, so the disorder is not passed 仕 om parent tO child. However, gene mutations, at least as we currently understand them in the context ofthis disorder, d0 not explain all incidences of narcolepsy. Other triggers remain to be identified. Narcolepsy is also not unique to humans, with numerous Other mammals expressing the disorder. There are at least three core symptoms that make up the disorder: ( 1 ) excessive daytime sleepiness, ( 2 ) sleep paralysis, and ( 3 ) cataplexy. The first symptom ofexcessive daytime sleepiness is often the most disruptive and problematic to the quality of day-to-day life for narco- leptic patients. lt involves daytime sleep attacks: overwhelming, utterly irresistible urges tO sleep at times when you want to be awake, such as working at your desk, driving, or eating a meal with family or friends. Having read that sentence, I suspect many of you are thinking,"Oh my goodness, I have narcolepsy!" That is unlikely. lt is far more prob- able that you are suffering 仕 om chronic sleep deprivation. About one ⅲ every 2 , 000 people suffers 仕 om narcolepsy, making it about as common

WHY WE SLEEP 5 5 Finally, and not to be left out of the descriptive REM-sIeep picture, iS the very reason for itS name: corresponding rapid eye movements. Your eyes remain still in their sockets during deep NREM sleep. * Yet electrodes that we place above and below your eyes tell a very differ- ent ocular story when you begin t0 dream: the very same story that Kleitman and Aserinsky unearthed in 1952 when observing infant sleep. During REM sleep, there are phases when your eyeballs will jag, with urgency, left-to-right, left-to-right, and so on. At first, scientists assumed that these rat-a-tat-tat eye movements corresponded tO the tracking Of visual experience ⅲ dreams. This is not true. lnstead, the eye movements are intimately linked with the physiological creation 0f REM sleep, and reflect something even more extraordinary than the passive apprehension 0f moving objects within dream space. This phe- nomenon is chronicled in detail in chapter 9. Are we the only creatures that experience these varied stages Of sleep? DO any other animals have REM sleep? DO they dream? Let us find out. *Oddly, during the transition 仕 om being awake into light stage 1 NREM sleep, the eyes will gently and very, very slowly start to roll in their sockets in synchrony, like 杁 ocular ballerinas pirouetting in perfect time with each other. lt is a hallmark indication that the onset 0f sleep is inevitable. If you have a bed partner, try observing their eyelids the next time they are drifting off to sleep. You will see the closed lids of the eyes deforming as the eyeballs roll around underneath. Parenthetically, should you choose t0 complete this sug- gested observational experiment, be aware ofthe potentialramifications. There is perhaps little else more di squieting than aborting one's transition into sleep, opening your eyes, and finding your partner's face looming over yours, gaze affixed.

2 4 2 MAITHEW WALKER ◆ Has insomnia at least three nights each week for more than three months ◆ Does not have any coexisting mental disorders or medical con- ditions that could otherwise cause what appears tO be insomnia What this really means ⅲ terms of boots-on-the-ground patient descriptions is the following chronic situation: difficulty falling asleep, waking 叩ⅲ the middle ofthe night, waking up t00 early ⅲ the morning, difficulty falling back to sleep after waking up, and feeling unrefreshed throughout the waking day. lfany ofthe characteristics ofinsomnia feel familiar to you, and have been present for several 襯 0 厩な , I suggest you consider seeking out a sleep medicine doctor. I emphasize a sleep med- icine doctor and not necessarily your GP, since GPs—superb as they 0ften are—have surprisingly minimal sleep training during the entirety 0f medical school and residency. Some GPs are understandably apt t0 prescribe a sleeping pill, which is rarely the right answer, as we will see in the next chapter. The emphasis on duration 0f the sleep problem (more than three nights a week, for more than three months) is important. AII of us ⅷⅡ experience difficulty sleeping every now and then, which may last just one night or several. That is normal. There is usually an obvious cause, S11Ch as work stress or a flare-up in a SOCial or romantic rela- tionship. Once these things subside, though, the sleep difficulty usually goes away. Such acute sleep problems are generally not recognized as ChroniC insomnia, since clinical insomnia reqmres an ongoing duration of sleep difficulty, week after week after week. Even with this strict definition, chronic insomnia is disarmingly common. Approximately one out Of every nine people you pass on the street will meet the strict clinical criteria for insomnia, which translates t0 more than 40 million Americans struggling t0 make it through their waking days due t0 wide-eyed nights. While the reasons remam unclear, insomnia iS almost tWice as common in women than in men, it iS unlikely that a simple unwillingness 0f men t0 admit sleep problems explains this very sizable difference between the VO sexes. Race and ethnicity also make a significant difference, with African Americans and Hispanic Americans suffering higher rates 0f insomnia than Cau-