W HY W E S L E E P twenty-four hours. Richardson, in his twenties, developed a sleep-wake cycle of between twenty-six and twenty-eight hours ⅲ length. That of KIeitman, ⅲ his forties, was a little closer to, but still longer than, twenty-four hours. Therefore, when removed 仕 om the external influ- ence 0f daylight, the internally generated "day" 0f each man was not exactly twenty-four hours, but a little more than that. Like an inaccu- rate wristwatch whose time runs long, with each passing (real) day in the outside world, Kleitman and Richardson began t0 add time based on their longer, internally ge nerated chronometry. Since our innate biological rhythm is not precisely twenty-four hours, but thereabouts, a new nomenclature was required: the circadian rhythm—that is, one that is 叩 pro. 襯砒 e or around, one day in length, and not precisely one day. * ⅲ the seventy-plus years since Kleitman and Richardson's seminal experiment, we have now determined that the average duration Of a human adult's endogenous circadian clock runs around twenty-four hours and fifteen minutes in length. NOt t00 far off the twenty-four-hour rotation of the Earth, but not the precise timing that any self-respecting Swiss watchmaker would ever accept. ThankfulIy, most of us don't live ⅲ Mammoth Cave, or the constant darkness it imposes. We routinely experience light 仕 om the sun that comes tO the rescue Of our imprecise, overrunning internal circadian clock. Sunlight acts like a manipulating finger and thumb on the side- dial ofan imprecise wrisfivatch. The light 0fthe sun methodically resets our inaccurate internal timepiece each and every day, "winding" us back t0 precisely, not approximately, twenty-four hours. lt is no coincidence that the brain uses daylight for this resetting purpose. Daylight is the most reliable, repeating signal that we have in our environment. Since the birth Of our planet, and every single day thereafter without fail, the sun has always risen in the morning and set in the evening. lndeed, the reason most living species likely *This phenomenon of an imprecise internal biological clock has now been consistently observed ⅲ many different species. However, it is not consistently long in all species, as it is ⅲ humans. For some, the endogenous circadian rhythm runs short, being less than twenty-four hours when placed ⅲ tOtal darkness, such as hamsters or squirrels. For others, such as humans, it is longer than twenty-four hours. tEven sunlight coming through thick cloud on a rainy day is powerful enough to help reset our biological clocks.

WHY WE SLEEP 2 6 7 One hundred years post-Edison, we now understand the biological mechanisms by which the electric lightbulbs managed t0 vet0 our nat- ural timing and quality ofsleep. The visible light spectrum—that which our eyes can see—runs the gamut 仕 om shorter wavelengths (approxl- mately 380 nanometers) that we perceive as COOler violets and blues, tO the longer wavelengths (around 700 nanometers) that we sense as warmer yellows and reds. Sunlight contains a powerful blend 0f all 0f these colors, and those in between ()s the iconic Pink Floyd album cover 0fD ん Side e M00 〃 illuminates ト 0 t0 speak]). Before Edison, and before gas and oillamps, the setting sun would take with it this full stream 0f daylight 仕 om our eyes, sensed by the twenty-four-hour clock within the brain (the suprachiasmatic nucleus, described ⅲ chapter 2 ). The loss of daylight informs our suprachias- matic nucleus that nighttime iS now in session; time tO release the brake pedal on our pineal gland, allowing it t0 unleash vast quantities of melatonin that signal t0 our brains and bodies that darkness has arrived and it is time for bed. Appropriately scheduled tiredness, f01 ー lowed by sleep, would normally occur several hours after dusk across our human COlle ctive. Electric light put an end to this natural order 0f things. lt redefined the meaning of midnight for generations thereafter. Artificial evening light, even that 0f modest strength, or lux, will f001 your suprachias- matic nucleus intO believing the sun has not yet set. The brake on mela- tonin, which should otherwise have been released with the timing 0f dusk, remains forcefully applied within your brain under duress 0f elec- tric light. The artificiallight that bathes our modern indoor worlds will there- fore halt the forward progress of biological time that is normally sig- naled by the evening surge ⅲ melatonin. Sleep in modern humans is delayed 仕 om taking off the evening runway, which would naturally occur some 、 vhere between eight and ten p. 1 れ . , just as Ⅵ℃ observe hunter-gatherer tribes. Artificial light ⅲ modern societies thus tricks us into believing night is still day, and does so using a physiological lie. The degree t0 which evening electric light winds back your internal twenty-four-hour clock is important: usually VO tO three hours each evening, on average. TO contextualize that, let's say you are reading

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-

MATTHEW WALKER 2 4 and body that the finish line of sleep has been reached. lt is time t0 call the race ofsleep over and allow active wakefulness tO return for the rest of the day. ln this regard, we human beings are "solar powered." Then, as light fades, so, t00 , does the solar brake pedal blocking melatonin. AS melatonin rises, another phase Of darkness is signaled and another sleep event is called t0 the starting line. You can see a typical profile ofmelatonin release in figure 2. lt starts a few hours after dusk. Then it rapidly rises, peaking around four a. m. Thereafter, it begins to drop as dawn approaches, falling t0 levels that are undetectable by early t0 midmormng. Figure 2 : The CycIe Of MeIatonin 50 4 (ewseld ll.u/id) L1!umepn 30 20 0 Noon 6 pm Midnight 6 am Noon Time HAVE RH 、 YTHM, WON'T TRAVEL The advent of the jet englne was a revolution for the mass transit of human beings around the planet. However, it created an unforeseen biO- 10 c calamity:jet planes offered the abilityto speed through time zones faster than our twenty-four-hour internal clocks could ever keep up with or adjust t0. Those jets caused a biological time lag: jet lag. As a result, we feel tired and sleepy during the day in a distant time zone because our internal clock still thinks it is nighttime. lt hasn't yet caught up. If that were not bad enough, at night, we are frequently unable tO initiate or maintain sleep because our internal clock now believes it tO be daytime. Take the example of my recent flight home to England 仕 om San Francisco. London is eight hours ahead of San Francisco. When I arrive

A T T H E windswept, mist-shrouded summit ( ろとんル ) , a barefoot woman prays at a erected mem- 0 Ⅳ ofall who died in lreland through violence," a tollthat contlnues in Northern lreland's religious strife. , , 二 0 」← 0 After a four- to five-hour-climb, the pilgrims 朝加 ) parade around a small oratory, reciting 'Hail Marys' and "Our Fathers. " They will enter the chapelto confess their sins, rece1Ve C01 1 れ umon , st. Patrick in an lrish-language hymn to "he who overcame the Druids. Archaeological evidence suggests that, even before Christiani ty, the mountal n , then known Crochan Aigh , 'the mount of the eagle," served as a holy site. 329

lt has been an emotional journey for us; finding out about each Other, discovering our needs and negotiat- ing our wants. lt has been filled with great hope, some despair, and much learning and wisdom. Our relation- ship together has grown as we discover the keys t0 success in love and our love for each Other deepens. ln talking with SO many women we found that we are not alone, that each one Of us is searching for love and self discovery. 100 Keys 地 Success ⅲ LO 怩 is about the yearning in each Of us and options that are available tO us on that journey. Four Of our daughters are women in their twenties and thirties and they also have contributed to the b00k, giving advice and feedback about the issues facing women today. We feel honored and proud that these four strong, individual, sometimes rebellious Ⅵー 01 れ en have helped us on our journey of understanding and clarity about love. Our deepest thanks to all the women who opened their hearts to us. We hope you, the reader will find success ⅲ love and use some 0f the keys in this b00k t0 help you on your journey 0 日 ove and personal discovery. Vicki Bennett lan Mathieson 3

3 4 MAITHEW WALKER hour circadian rhythm and the sleepiness signal of adenosine—are independent, and can be decoupled 仕 om their normallockstep. Let's consider figure 7 , showing the same forty-eight-hour slice of time and the ⅲ℃ factors in question: the twenty-four-hour circadian rhythm and the sleep pressure signal of adenosine, and how much dis- tance there is between them. ⅲ this scenario, our volunteer is going to stay awake night and day. As the night of sleep deprivation marches forward, the sleep pressure ofadenosine (upper line) rises pro- gressively, like the rising water level in a plugged sink when a faucet has been left on. lt will not decline across the night. lt cannot, since sleep iS absent. Figure 7 : The Ebb and 日 ow Of Sleep Deprivation Overpowering urge tO sleep (SIeep drive) Process-S ー、 Strong urge tO sleep Weak urge tO sleep Process-C Circadian (Wake drive) 7 am awake 11 pm 7am awake sleep 11 pm 70E By remaming awake, and blocking access to the adenosine drain that sleep opens up, the brain is unable to rid itselfofthe chemical sleep pressure. The mounting adenosine levels continue tO rise. This should mean that the longer you are awake, the sleepier you feel. But that's not true. Though you ⅶⅡ feel increasingly sleepy throughout the nighttime phase, hitting a lowpoint in your alertness around five tO six a. m. , there- after, you'll catch a second wind. How is this possible when adenosine levels and corresponding sleep pressure continue t0 increase? The answer resides with your twenty-four-hour circadian rhythm, which offers a brief period of salvation from sleepiness. Unlike sleep pressure, your circadian rhythm pays no attention tO whether you are asleep or awake. lts slow, rhythmic countenance continues to fall and rise strictly on the basis Of what time of night or day it is. NO matter what state Of adenosine sleepiness pressure exists within the brain, the

ージ夜郎に流されて、辛判官に贈る 一五一ペ Long ago, among the flowers and willows, We sat drinking together at Ch' ang-an ・ Five Barons and Seven Grandees 舅「 e e Of our company, But when some wild stroke was afoot lt was we whO led it, yet boisterous though we were ln the arts and graces Of life we could hOld our own 「 ith any dandy in the town—・ ln the days when there was youth in your cheeks And I was still not 01d. 「 e galloped to the brothels, cracking our gilded whips, 舅「 e sent in our writings tO the Palace Of the Unicorn, Girls sang tO us and danced hour by hour on tortoise ・ shell mats. 「 e thought, ou and I, that it would be always like this. How should we know the grass would stir and dust rise on the wind ~ Sudden1y foreign horsemen were at the Hsien-ku Pass Just when the blossom at the palace 0 ( Ch'in was opening on the sunny boughs. Now I, unhappy, am on my way t0 banishment at Yeh ・ lang' 「 ondering if a day will come when the Golden COCk

AN O P E N H E A RT each leading to another moment of knowing. The stream of such moments Of conscrousness goes from hour t0 hour, 仔 om day [ 0 day, om year to year, and even, according to the Buddhist view, from lifetime to lifetime. Though our bOdy cannot accompany us once our li force is exhausted, the moments Of consclousness contlnue, through death and eventually into the next 1 飛 , whatever form it may take. Each one Of us possesses such a stream Of conscrousness. 、 it iS both beginningless and endless. Nothing can stop it. ln this sense lt IS unlike emotlons SLICh as anger or attachment, which can be made to cease by applying antidotes. Further- more, the essential nature Of the mind iS said tO be pure; its pollutants are removable, making the continuatlon of this purified mind eternal. Such a mind, free of pollution, is a Buddha's truth body. lfwe contemplate the state of 応Ⅱ enlightenment in this way, our appreciation of the Buddha's magnitude grows, as does our faith. As we recognize the qualities of a Buddha, our aspiration tO attain this state 1ntensifies.We come tO appreciate the value and necessity Of being able [ 0 emanate different forms in order [ 0 assist infinite beings. This gives us the strength and determrnation to achieve the enlight- ened mind. 1 6 8

266 MATTHEW WALKER ronmental light and, with it, our wake and sleep phases. We, and not the rotating mechanics ofplanet Earth, would now decide when it was "night" and when it was "day." We are the only species that has managed to light the night t0 such dramatic effect. Humans are predominantly visual creatures. More than a third of our brain is devoted tO processing visual information, far exceeding that given over t0 sounds or smells, or those supporting language and movement. For early HOI れ 0 sapiens, most Of our activities have ceased after the sun set. They had tO, as they were predicated on vision, supported by daylight. The advent of fire, and its limited halo of light, offered an extension to post-dusk activities. But the effect was mod- est. ln the early-evening glow Of firelight, nominal social activities such as singmg and storytelling have been documented ⅲ hunter-gatherer tribes like the Hadza and the San. Yet the practical limitations of fire- light nullified any significant influence on the timing of our sleep-wake p atterns. Gas- and oil-burning lamps, and their forerunners, candles, offered a more forceful influence upon sustained nighttime activities. Gaze at a Renoir painting of nineteenth-century Parisian li 他 and you will see the extended reach of artificial light. SpiIIing out of homes and onto the streets, gas lanterns began bathing entire city districts with illu- mination. ln this moment, the influence Of man-made light began its reengmeering 0f human sleep patterns, and it would only escalate. The nocturnal rhythms 0f whole societies—not just individuals or single families—became quickly subject to light at night, and so began our advancing march toward later bedtimes. For the suprachiasmatic nucleus—the master twenty-four-hour clock Of the brain—the worst was yet to come. Edison's Manhattan power station enabled the mass adoption 0f incandescent light. Edi- son did not create the first incandescent lightbulb—that honor went to the EngIish chemist Humphry Davy ⅲ 1802. But ⅲ the mid-1870s, Edison Electric Light Company began developing a reliable, mass- marketable lightbulb. lncandescent light bulbs, and decades later, fluorescent light bulbs, guaranteed that modern humans would no longer spend much of the night in darkness, as we had for millennia p ast.