2 ー 8 MATHEW WALKER discuss the damage that early schOOl start times are having on our teen- agers. MOSt significant is the issue Of sunrise schOOl bus schedules that selectively deprive our teenagers Of that early-morning slumber, just at the moment in their sleep cycle when their developing brains are about to drink in most oftheir much-needed REM sleep. We are bankrupting their dreams, in SO many different ways.

9 6 MATTHEW WALKER These three key changes are: ( 1 ) reduced quantity/quality, ( 2 ) reduced sleep efficiency, and ( 3 ) disrupted timmg of sleep. The postadolescent stabilization of deep-NREM sleep ⅲ your early twenties does not remaln very stable for very long. Soon—sooner than you may imagme or wish—comes a great sleep recession, with deep sleep being hit especially hard. ln contrast t0 REM sleep, which remains largely stable ⅲ midlife, the decline 0f deep NREM sleep is already under way by your late twenties and early thirties. As you enter your fourth decade of life, there is a palpable reduc- tion in the electrical quantity and quality ofthat deep NREM sleep. You obtain fewer hours of deep sleep, and those deep NREM brainwaves become smaller, less powerful, and fewer ⅲ number. passing intO your mid- and late forties, age ⅷⅡ have stripped you of 60 to 70 percent 0f the deep sleep you were enJoynng as a young teenager. By the time you reach seventy years 01d , you ⅶⅡ have lost 80 t0 90 percent 0f your youthful deep sleep. Certainly, when we sleep at night, and even when we wake in the morning, most Of us dO not have a good sense Of our electrical sleep quality. Frequently this means that many seniors progress through their later years not fully realizing how degraded their deep-sleep quantity and quality have become. This is an important point: it means that elderly individuals fail tO connect their deterioration in health with their deterioration ⅲ sleep, despite causallinks between the れ MO having been known to scientists for many decades. Seniors therefore complain about and seek treatment for their ん〃ん issues when visiting their GP, but rarely ask for help with their equally prob- lematic sleep issues. AS a result, GPs are rarely motivated tO address the problematic sleep in addition t0 the problematic health concerns of the older adult. To be clear, not all medical problems of aging are attributable t0 poor sleep. But far more of our age-related physical and men- tal health ailments are related to sleep impairment than either we, or many doctors, truly realize or treat seriously. Once again, I urge an elderly individual who may be concerned about their sleep not t0 seek a sleeping pill prescription. lnstead, I recommend you first explore the effective and scientifically proven non-pharmacological

3 3 0 MATHEW WALKER Reinforced day after day, month after month, and ultimately year after year, this nudge could change many people's sleep neglect for the better. l'm not so naive to think it would be a radical change, but ifthis increased your sleep amount by just fifteen tO twenty minutes each night, the science indicates that it would make a significant differ- ence across the life span and save trillions of dollars within the global economy at the population level, tO name but ℃ benefits. lt could be one Of the most powerful factors in a future vision that shifts 仕 om a model 0f sick care (treatment), which is what we do now, to health care (prevention)—the latter aiming tO stave Off a need for the former. Prevention iS far more efficient than treatment, and COStS far less in the long run. GOing even further, what ifwe moved 仕 om a stance 0f4 〃を〃 cs (). e. , here is your past and/or current sleep and here is your past and/or cur- rent body weight) to that of forward-lookingpredictalytics? To explain the term, let me go back to the smoking example. There are efforts to create predictalytics apps that start with you taking a picture 0f your own face with the camera ofyour smartphone. The app then asks you hOW many cigarettes you smoke on average a day. Based on scientific data that understand how smoking quantity impacts outward health features such as bags under your eyes, wrinkles, psonasis, thinmng hair, and yellowed teeth, the app predictively modifies your face on the assumption Ofyour continued smoking, and does SO at different future time points: one year, two years, five years, ten years. The very same approach could be adopted for sleep, but at many different levels: outward appearance as well as inward brain and body health. For example, we could show individuals their increasing risk (alb eit non-determini stic) Of conditions such as A レ heimer's di sease or certain cancers if they continue sleeping t00 li 田 e. Men could see projections on hOW much their testicles will shrink or their testoster- one level will drop should their sleep neglect continue. Similar risk pre- dictions could be made for gains ⅲ body weight, diabetes, or immune impalrment and infection. Another example involves offering individuals a prediction of when they should or should not get their flu shot based on sleep amount in the week prior. You Ⅱ recall 仕 om chapter 8 that getting four to six

ー 4 2 MATTHEW WALKER bus. AII three vehicles traveled through a ditch and continued mov- ing, at which point the imploded Pontiac became engulfed in flames. The schOOl bus rotated counterclockwise and kept traveling, now on the opposite side 0f the road, back-to-front. lt did so for 328 feet until it went off the road and collided with a thick grove 0f trees. Three 0f the nine children in the bus were ejected through the windows upon impact. AII seven passengers ⅲ the Pontiac were killed, as was the bus driver. The truck driver and nine children in the bus sustained seri- OIIS llljurles. The trucker was a qualified and legally licensed driver. AII to colo tests performed on his blOOd were negative. However, it later emerged that he had been awake for thirty-four hours straight and had fallen asleep at the wheel. All Ofthe Pontiac's seven occupants whO died were children or adolescents. Five of the seven were children ⅲ the Pontiac car were 仕 om a single family. The Oldest occupant was a teenager, whO had been legally driving the car. The youngest occupant was a baby of j ust ⅲ℃ n months 01d. There are many things that I hope readers take away 仕 om this book This is one of the most important: if you are drowsy while driving, please, please 立 . lt is lethal. TO carry the burden 0f another's death on your shoulders is a terrible thing. Don't be misled by the many inef- fective tactics people will tell you can battle back against drowsiness while driving.* Many of us think we can overcome drowsiness through sheer force ofwill, but, sadly, this is not true. TO assume otherwise can jeopardize your life, the lives 0f your family or friends in the car with you, and the lives ofother road users. Some people only get one chance to fall asleep at the wheel before losing their life. lfyou notice yourselffeeling drowsy while driving, or actually falling asleep at the wheel, stop for the night. If you really must keep going— and you have made that judgment in the life-threatening context it genuinely poses—then pull 0ff the road int0 a safe layby for a short time. Take a brief nap (twenty t0 thirty minutes).When you wake up, *Common myths that are Of no use in helping tO overcome drowsiness while driving include: turning up the radio, winding down the car window, blowing cold air on your face, splashing cold water on your face, talking on the phone, chewing gum, sl 叩 ping yourself, pinching yourself, punching yourself, and promising yourself a reward for staying awake.

W HY W E S L E E P ー 2 7 仕 om that for textbook-like memory. Rather than a transfer 仕 om short- tO long-term memory required for saving facts, the motor memories had been shifted over t0 brain circuits that operate below the level 0f consciousness. As a result, those skill actions were now instinctual hab- its. They flowed out of the body with ease, rather than feeling effortful and deliberate. Which is to say that sleep helped the brain automate the movement routines, making them second nature—effortless— precisely the go 0f many an Olympic coach when perfecting the skills of their elite athletes. My final discovery, in what spanned almost a decade 0f research, identified the 与甲 e 0f sleep responsible for the overnight motor-skill enhancement, carrying with it societal and medical lessons. The increases in speed and accuracy, underpinned by efficient automaticity, were directly related t0 the amount 0f stage 2 NREM, especially in the last two hours 0f an eight-hour night 0f sleep (). g. , 仕 om five t0 seven a. m. , should you have fallen asleep at eleven p. m. ). lndeed, it was the number 0f those wonderful sleep spindles in the last 0 hours 0f the late morning—the time ofnight with the richest spindle bursts ofbrain- wave activity—that were linked with the offline memory boost. More striking was the fact that the increase Of these spindles after learning was detected only ⅲ regons 0f the scalp that sit above the motor cortex (just in front Ofthe crown Ofyour head), and not in Other areas. The greater the local increase in sleep spindles over the part 0f the brain we had forced t0 learn the motor skill exhaustively, the bet- ter the performance upon awakening. Many other groups have found a similar "local-sleep"-and-learning effect. When it comes t0 motor-skill memories, the brainwaves Of sleep were acting like a good masseuse— you still get a んⅡ body massage, but they will place special focus on areas 0f the b0dy that need the most help. ln the same sleep spin- dles bathe 記 1 parts 0fthe brain, but a disproportionate emphasis will be placed on those parts 0f the brain that have been worked hardest with learning during the day. Perhaps more relevant tO the modern world is the time-of-night effect we discovered. Those last い VO hours ofsleep are precisely the win- dow that many ofus feel it is okay t0 cut short tO get a jump start on the day. As a result, we miss out on this feast oflate-morning sleep spindles.

274 MAITHEW WALKER ing their sleep doused with alcohol on the third night still resulted in almost the same degree of amnesia—40 percent ofthe knowledge they had worked so hard t0 establish on day 1 was forgotten ・ The overnight work 0f REM sleep, which normally assimilates com- plex memory knowledge, had been interfered with by the alcohol• More surprising, perhaps, was the realization that the brain is not done pro- cessing that knowledge after the first night 0f sleep. Memories remain perilously vulnerable t0 any disruption 0f sleep (including that 仕 om alcohol) even up to three nights after learning, despite れ vo 応Ⅱ nights 0f natural sleep prior. Framed practically, let's say that you are a student cramming for an exam on Monday. Diligently, you study ofthe previous Wednesday. Your friends beckon you to come out that night for drinks, but you ow how important sleep is, so you decline. On Thursday, friends 凾 n ask you to grab a few drinks ⅲ the evening but t0 be safe, you turn them down and sleep soundly a second ⅲ t. Finally, Friday rolls around—now three nights afteryour learning session—and everyone is heading out for aparty and drinks. Surely, after being so dedicated to slumber across the first two nights after learning, you can now cut loose, knowing those memories have been safely secured and ftllly processed within your memory banks Sadly, not so. Even now, C0h01 consumption will wash away much ofthat which you learned and can abstract by blocking your REM sleep. HOW long is it before those new memories are finally safe? Ⅵ厄 actu- ally d0 not yet know, though we have studies under way that span many weeks. What we do know is that sleep has not finished tending to those newly planted memories by night 3. I elicit audible groans when I pres- ent these findings t0 my undergraduates in lectures. The politically incorrect advice I would ()f course never) give is this: go t0 the pub for a drink in the morning. That way, the a1C0h01 will be out 0fyour system before sleep. Glib advice aside, what is the recommendation when it comes tO sleep and C0h01 ? lt is hard not t0 sound puritanical, but the evidence is so strong regarding alcohol's harmful effects on sleep that t0 d0 oth- erwise would be doing you, and the science, a disservice. Many people enjoy a glass Ofwine with dinner, even an aperitifthereafter. But it takes your liver and kidneys many hours t0 degrade and excrete that alco-

CHAPTER 8 Cancer, Heart Attacks, and a Shorter Life e 孕つ孕 r ル砒 / 0 〃〃 d the 召 0 I was once fond of saying,"Sleep is the third pillar 0f good health, alongside diet and exercise. " I have changed my tune. Sleep is more than a pillar; it is the foundation on which the 0ther two health bas- tions sit. Take away the bedrock 0f sleep, or weaken it just a little, and careful eating or physical exercise become less than effective, as we shall see. Yet the insidious impact of sleep loss on health runs much deeper. Every rnaJ0r system, tissue, and organ 0f your b0dy suffers when sleep becomes short. NO aspect ofyour health can retreat at the sign 0f sleep loss and escape unharmed. Like water 仕 om a burst pipe ⅲ your home, the effects of sleep deprivation 、ⅵⅡ seep into every nook and cranny of biology, down into your cells, even altering your most fundamental self—your DNA. Widening the lens of focus, there are more than twenty large-scale epidemiological studies that have tracked millions ofpeople over many decades, all 0f which report the same clear relationship: the shorter your sleep, the shorter your life. The leading causes ofdisease and death in developed nations—diseases that are crippling health-care systems, such as heart disease, obesity, dementia, diabetes, and cancer—all have recognized causal links t0 a lack 0f sleep. This chapter describes, uncomfortably, the many and varied ways in which insufficient sleep proves ruinous t0 all the major physiological systems Of the human bOdy: cardiovascular, metabolic, immune, reproductive.

CHAPTER 13 iPads, Factory Whistles, and Nightcaps ルカ 4 な & 02 〃 g 物リ尹 0 襯 e 孕加 g ? Many 0f us are beyond tired. Why? What, precisely, about modernity has SO perverted our otherwise instinctual sleep patterns, eroded our freedom to sleep, and thwarted our ability t0 d0 so soundly across the night? For those of us wh0 d0 not have a sleep disorder, the reasons underlying this state ofsleep deficiency can seem hard t0 pinpoint—or, if seemingly clear, are erroneous. Beyond longer commute times and "sleep procrastination" caused by late-evening television and digital entertainment—both 0f which are not unimportant ⅲ their top-and-tail snipping Of our sle ep time and that ofour children—five key factors have powerfully changed how much and how well we sleep: ( 1 ) constant electric light as well as LED light, ( 2 ) regularized temperature, ( 3 ) caffeine (discussed in chapter 2 ) , ( 4 ) alcohol, and ( 5 ) a legacy ofpunching time cards. lt is this set 0f soci- etally engmeered forces that are responsible for many an individual's mistaken beliefthat they are suffering 仕 om medical insomnia. THE DARK SIDE OF MODERN 凵 GHT At 255 ー 257 pearl street, in Lower Manhattan, not far 仕 om the Br00k- lyn Bridge, is the site of arguably the most unassuming yet seismic shift ⅲ our human history. Here Thomas Edison built the first power- generating station tO support an electrified society. For the first the human race had a truly scalable method 0f unbuckling itself 仕 om our planet's natural twenty-four-hour cycle 0f light and dark With a proverbial flick 0fa switch came a whimsical ability t0 control our envl-

WHY WE SLEEP 2 0 5 no disguise. Dream sources are transparent—clear enough for anyone tO identify and recogmze without the need for an interpreter. DO DREAMS HAVE A FUNCTION? Through a combination Ofbrain activity measures and rigorous experl- mental testing, we have finallybegun to develop a scientific understand- ing 0f human dreams: their form, content, and the waking source(s). There is, however, something missing here. None of the studies that I have described so far proves that dreams have any åtnction. REM sleep, 仕 om which principal dreams emerge, certainly has many functions, as we have discussed and will continue to discuss. But do dreams them- selves, above and beyond REM sleep, actually do anything for us? As a matter 0f scientific fact, yes, they do.

2 6 0 MATHEW WALKER me tO read, I must confess). Several years after de Manacéine's studies, ltalian researchers described equally lethal effects of total sleep depri- vation in dogs, adding the observation Of neural degeneration in the brain and spinal cord at postmortem. lt tookanother hundred years after the experiments ofde Manacéine, and the advancements in precise experlmental laboratory assessments, before the scientists at the University of Chicago finally uncovered why life ends so quickly in the absence 0f sleep. Perhaps you have seen that small plastic red box on the walls ofextremely hazardous work environ- ments that has the following words written on the front: "Break glass ⅲ case Of emergency." lfyou impose a tOtal absence Of sleep on an organ- ism, rat or human, it indeed becomes an emergency, and you will find the biological equivalent of this shattered glass strewn throughout the brain and the b0 t0 信 t effect. This we finally understand. NO, WAIT—YOU ONLY NEED 6.75 HOURS OF SLEEP! Reflecting on these deathly consequences of long-term/chronic and short-term/acute sleep deprivation allows us tO address a recent con- troversy ⅲ the field 0f sleep research—one that many a newspaper, not tO mention some scientists, apprehended incorrectly.. The study in question was conducted by researchers at the University 0f California, LOS Angeles, on the sleep habits of specific pre-industrial tribes. Using wristwatch activity devices, the researchers tracked the sleep Of three hunter-gatherer tribes that are largely untouched by the ways ofindus- trial modernity: the Tsimané people in South America, and the San and Hadza tribes in Africa, which we have previously discussed. Assessing sleep and wake times day after day across many months, the findings were thus: tribespeople averaged just 6 hours of sleep in the summer, and about 7.2 hours of sleep in the winter. Well-respected media outlets touted the findings as proof that human beings do not, after 心 , need a んⅡ eight hours 0f sleep, some suggesting we can survive just fine on six hours or less. For example, the headline ofone prominent US newspaper read: