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1. Why We Sleep The New Science of Sleep and Dreams

6 4 MATTHEW WALKER never comes close t0 getting back the sleep it has lost. This is true for total sleep time, just as it is for NREM sleep and for REM sleep. That humans (and other species) can never "sleep back" that which we have previously lost is one 0fthe most important take-homes 0fthis b00k, the saddening consequences of which I ⅵⅡ describe ⅲ chapters 7 and 8. IF ONLY HUMANS COULD A third striking difference in sleep across the animal kingdom is the ル 2 ⅲ which we 1 d0 it. Here, the diversity is remarkable and, in some cases, almost impossible tO believe. Take cetaceans, such as dolphins and whales, for example. Their sleep, ofwhich there is only NREM, can be unihemispheric, meaning they ⅶⅡ sleep with half a brain at a time! One halfofthe brain must always stay awake t0 maintain life-necessary movement ⅲ the aquatic enuronment. But the Other half Of the brain will, at times, fall into the most beautiful NREM sleep. Deep, powerful, rhythmic, and slow brainwaves Ⅱ drench the entirety of one cerebral hemisphere, yet the other halfofthe cerebrum ⅶⅡ be bristling with fre- netic, fast brainwave activity, fully awake. This despite the fact that both hemispheres are heavily wired together with thick crisscross fibers, and sit mere millimeters apart, as in human brains. Of course, both halves of the dolphin brain can be, and frequently are, awake at the very same time, operating ⅲ unison. But when it is time for sleep, the れ VO sides Of the brain can uncouple and operate independently, one side remaming awake while the Other side snoozes away. After this one half of the brain has consumed its fill 0f sleep, they switch, allowing the previously vigilant half 0f the brain t0 enJOY a well-earned period of deep NREM slumber. Even with half 0f the brain asleep, dolphins can achieve an impressive level ofmovement and even S01 れ e vocalized communication. The neural engineering and tricky architecture required tO accom- plish this staggering trick of oppositional "lights-on, lights-off" brain activity is rare. Surely M0ther Nature could have found a way t0 avoid sleep entirely under the extreme pressure Of nonstop, 24 / 7 aquatic movement. Would that not have been easier than masterminding a convoluted split-shift system between brain halves for sleep, while

2. Why We Sleep The New Science of Sleep and Dreams

Acknowledgments The staggering devotion of my fellow sleep scientists in the field, and that of the students in my own laboratory, made this book possible. Without their heroic research efforts, it would have been a very thin, uninformative text. Yet scientists and young researchers are only half of the facilitating equation when it comes to discovery. The invaluable and willing participation of research subjects and patients 引 lows fun- damental scientific breakthroughs to be uncovered. I offer my deepest gratitude to ofthese individuals. Thank you. Three other entities were instrumental ⅲ bringing this book to life. First, my inimitable publisher, Scribner, who believed in this book and its 10 mission to change society. Second, my deftly skilled, inspiring, and deeply committed editors, Shannon Welch and Kathryn Belden. Third, my spectacular agent, sage writing mentor, and ever-present literary guiding light, Tina Bennett. My only hope is that this book represents a worthy match for all you have given tO me, and it.

3. Why We Sleep The New Science of Sleep and Dreams

W H Y W E S L E E P 3 2 7 We could even use this same light-manipulation idea to apply a slight nudge in someone's sleep-wake rhythm within a biologically reasonable range (plus or minus thirty t0 forty minutes), should they desire, grad- ually moving it earlier or later. For example, if you have an unusually early morning meeting in the middle ofthe workweek, this technology, synched to your online calendar, would gradually begin shifting you (your circadian rhythm) to a slightly earlier bed and rise time starting on Monday. This way, that early-morning rise time on Wednesday won't be as miserable, or cause such biological turmoil within your brain and body. This would be equally, ifnot more, applicable ⅲ helping individu- als overcome jet lag when traveling between time zones, 引 1 dispensed through LED-emitting personal devices that people already travel with—phones, tablets, 1 叩 top computers. Why stop at the home environment or in the infrequent circum- stance ofjet lag? Cars can adopt these same lighting solutions t0 help manipulate alertness during morning commutes. Some Of the highest rates Of drowsy-driving accidents occur during mornings, especially early mornings. What if car cockpits could be bathed in blue light dur- ing early-morning commutes? The levels would have t0 be tempered SO as not tO distract the driver or Others on the road, but you'll recall 仕 om chapter 13 that one does not need especially bright light (lux) tO have a measurable impact Of melatonin suppression and enhanced wakefulness. This idea could be particularly helpful ⅲ those parts of the Northern and Southern Hemispheres during their respective win- ter mornings where this issue is most problematic. ln the workplace, for those lucky enough t0 have their own office, lighting rhythm could be custom fit tO the occupant using the same principles. But even cubi- cles, which are not so different from the cell 0f a car, could be person- ally tailored in this light-dependent manner, based on the individual sitting ⅲ that cubicle. How much benefit such changes would make remains t0 be proven, but I can already tell you 0f some data 仕 om ever-sleep-sensitive NASA, with which I worked on sleep issues early ⅲ my career. Astronauts on the lnternational Space Station travel through space at 17 , 500 miles per hour and complete an orbit Of the Earth once every ninety tO one hundred minutes. As a result, they experience "daylight" for about fifty

4. Why We Sleep The New Science of Sleep and Dreams

CHAPTER 10 Dreaming as Overnight Therapy lt was long thought that dreams were simply epiphenomena 0f the stage ofsleep (REM) from which they emerge. TO illustrate the concept ofepiphenomena, let's consider the lightbulb. The reason we construct the physical elements 0f a lightbulb—the glass sphere, the coiled wire element that sits inside, the screw-in elec- trical contact at the base—is tO create light. That is the function Ofthe lightbulb, and the reason we designed the apparatus t0 begin with. However, a lightbulb 引 so produces heat. Heat is not the function ofthe lightbulb, nor is it the reason we 0 ⅱ n 引 ly fashioned it. lnstead, heat is simply what happens when light is generated ⅲ this way. lt is an unin- tended by-product Of the operation, not the true function. Heat is an epiphenomenon in thiS case. Similarly, evolution may have gone tO great lengths tO construct the neural circuits in the brain that produce REM sleep and the functions that REM sleep supports. However, when the (human) brain produces REM sleep in this specific way, it may also produce this thing we call dreaming. Dreams, like heat 仕 om a lightbulb, may serve no function. Dreams may simply be epiphenomena of no use or consequence. They are merely an unintended by-product 0f REM sleep. Rather a depressing thought, isn't it? l'm sure many 0f us feel that our dreams have meamng and some useful purpose. TO address this stalemate, exploring whether dreaming, beyond the stage 0f sleep it emerges 仕 om , has true purpose, scientists began by defining the functions 0fREM sleep. Once those functions were known, we could then examine whether the dreams that accompany REM

5. Why We Sleep The New Science of Sleep and Dreams

2 0 8 MATTHEW WALKER cocktail 0f your brain that takes place during REM sleep. Concentra- tions Of a key stress-related chemical called noradrenaline are com- pletely shut 0ff within your brain when you enter this dreaming sleep state. ln fact, REM sleep is the only time during the twenty-four-hour period when your brain is completely devoid of this anxiety-triggenng molecule. Noradrenaline, so known ・ as norepinephrine, is the brain equivalent to a body chemical you already ow and have felt the effects 0 adrenaline (epinephrine). Previous MRI studies established that key emotion- and memory- related structures of the brain are reactivated during REM sleep, as we dream: the amygdala and emotion-related regions ofthe cortex, and the key mnemomc center, the hippocampus. N0t only did this suggest the possibility that emotion-specific memory processing was possible, if not probable, during the dreaming state, but now we understood that this emotional memory reactivation was occurring ⅲ a brain free of a key stress chemical. I therefore wondered whether the brain dur- ing REM sleep was reprocessing upsetting memory expenences and themes in this neurochemically calm (low noradrenaline), "safe" dream- ing brain environment. ls the REM-sleep dreaming state a perfectly designed nocturnal soothing balm—one that removes the emotional sharp edges 0f our daily lives? lt seemed so 仕 om everything neurobiol- ogy and neurophysiology was telling us (me). If so, we should awake feeling better about distressing events ofthe day(s) prior. This was the theory 0f overnight therapy. lt postulated that the process 0f REM-sleep dreaming accomplishes れ vo critical goals: ( 1 ) sleeping t0 remember the details 0f those valuable, salient expen- ences, integrating them with existing knowledge and putting them int0 autobiographical perspective, yet ( 2 ) sleeping t0 弘 or dissolve, the visceral, painful emotional charge that had previously been wrapped around those memories. If true, it would suggest that the dream state supports a form Of introspective life review, tO therapeutic ends. Think back to your childhood and try to recall some ofthe strongest memories you have. What you will notice is that almost all ofthem will be memories 0f an emotional nature: perhaps a particularly frighten- ing experience 0f being separated 仕 om your parents, or almost being hit by a car on the street. 用 SO notice, however, that your recall ofthese

6. Why We Sleep The New Science of Sleep and Dreams

WHY WE SLEEP 2 0 ー rather like email encryption wherein the message is cloaked with a code).Without the decryption key, the content of the email cannot be read. Freud felt that he had discovered the decryption key to everyones dreams, and for many of his affluent Viennese patients, he offered the paid service 0f removing this disguise and revealing to them the 0 ⅱ - nal message content oftheir dreams. The problem, however, was the lack of any clear predictions from Freud's theory. Scientists could not design an experiment that would test any tenets ofhis theory in order to help support or falsify it. lt was Freud'S gemus, and hiS simultaneous downfall. science could never prove him wrong, which is why Freud continues tO cast a long shadow on dream research to this day.. But by the very same token, we could never prove the theory right. A theory that cannot be discerned true or false in this way ⅶⅡ always be abandoned by science, and that is pre- cisely what happened to Freud and his psychoanalytic practices. AS a concrete example, consider the scientific method of carbon dating, used to determine the age of an organic object like a fossil. To validate the method, scientists would have the same fossil analyzed by several different carbon-dating machines that operated on the same underlying principle. lfthe method was scientifically robust, these inde- pendent machines should return the same value ofthe fossil's age. If they d0 not, the method must be flawed, as the data is inaccurate and cannot be replicated. The method ofcarbon dating was shown by this process to be legit- imate. N0t so for the Freudian psychoanalytic method of dream inter- pretation. Researchers have had different Freudian psychoanalysts interpret the same dream 0f an individual. lfthe method was scientifi- cally reliable, with clear structured rules and metrics that the thera- pists could apply, then their respective interpretations Of this dream should be the same—or at least have some degree of similarity ⅲ the extracted meaning they return. lnstead, the psychoanalysts all gave remarkably different interpretations Of this same dream, without any statistically significant similarity between them. There was no consis- tency. You cannot place a "QC"—quality control—sticker on Freudian psychoanalysis. A cynical criticism 0f the Freudian psychoanalytic method is

7. Why We Sleep The New Science of Sleep and Dreams

2 2 MATTHEW WALKER ln this regard, a societal change is needed, offering accommodations not dissimilar t0 those we make for other physically determined differences (). g. , sight impaired). We require more supple workschedules that better adapt to 1 chronotypes, and not just one ⅲ its extreme. You may be wondering why M0ther Nature would program this variability across people. As a social species, should we not all be syn- chronized and therefore awake at the same time tO promote maximal human interactions? Perhaps not. AS we'll discover later in this bOOk, humans likely evolved t0 co-sleep as families or even whole tribes, not alone or as couples. Appreciating this evolutionary context, the benefits of such genetically programmed variation in sleep/wake timing preferences can be understood. The night owls ⅲ the group would not be going tO sleep until one or MO a. m. , and not waking until nine or ten a. m. The morning larks, on the other hand, would have retired for the night at nine p. m. and woken at five a. m. Conse- quently, the group as a whole is only collectively vulnerable (). e. , every person asleep) for just four rather than eight hours, despite everyone still getting the chance for eight hours 0f sleep. That's potentially a 50 percent increase in survival fitness. Mother Nature W0111d never pass on a biological trait—here, the useful variability in when individuals within a collective tribe go t0 sleep and wake up—that could enhance the survival safety and thus fitness 0f a species by this amount. And so she hasn't. M E LATO N ー N Your suprachiasmatic nucleus communicates itS repeating signal Of night and day t0 your brain and body using a circulating messenger called melatonin. Melatonin has other names, t00. These include "the hormone Of darkness" and "the vampire hormone." NOt because it is sinister, but simply because melatonin is released at night. lnstructed by the suprachiasmatic nucleus, the rise in melatonin begins soon after dusk, being released into the bloodstream 仕 om the pineal gland, an area situated deep in the back Of your brain. Melatonin acts like a powerful bullhorn, shouting out a clear message t0 the brain and body:"lt's dark, it's dark!" At this moment, we have been served a writ

8. Why We Sleep The New Science of Sleep and Dreams

WHY WE SLEEP processes Of sleep operate. that certain conceptual parallels offer useful analogies to comprehend how the biological A brain cannot be said tO be the equivalent Of a computer, nor vice versa. lt is simply There are abstract similarities, yes, but there are many clear differences, large and small. human brain, or even its functions Of learning and memory, operates as a computer does. *The literal-minded reader should not take this an 0 to suggest that I believe the Ofbroader societal relevance, the concentration ofNREM-sleep spin- worldwide. pharmacological methods for improving sleep ⅲ agmg populations elderly, further compelling researchers such as myselfto find new, non- reason for medicine tO take more seriously the sleep complaints Of the list 0f facts we presented. This sleep and learning link is yet one more vidual the next day, making it more difficult for them to memorize the on a particular night, the lower the learning capacity ofthat older indi- we found: the fewer the number of spindles an elderly brain produced memory capacity. We conducted the study, and that is precisely what they have not received as much overnight refreshment Of short-term for them tO cram new facts intO their hippocampus the next day, since spindles an older adult has on a particular night, the harder it should be suffering a 40 percent deficit. This led t0 a prediction: the fewer sleep t0 generate sleep spindles t0 the same degree as young, healthy adults, We have found that seniors (aged sixty to eighty years old) are unable Our recent work on this topic has returned tO the question Of aging ・ night learning ability come the next morning. spindles an individual has at night, the greater the restoration Of over- a んⅡ night of sleep and replicated the same finding: the more sleep We and other research groups have since repeated this study across new facts could begin again, anew, the following day. imprinted experiences tO a more permanent safe hOld. The learning Of information within the hippocampus, having relocated yesterday's space. Participants awoke with a refreshed capacity t0 absorb new replemshing this short-term information repository with plentiful free ln doing so, sleep had delightfully cleared out the hippocampus, depot (the hippocampus) t0 a long-term secure vault (the cortex). that was shifting fact-based memories 仕 om the temporary storage an electrical transaction occurring in the qlllet secrecy Of sleep: one memory hard drive). * ln that moment, we had just become privy t0

9. Why We Sleep The New Science of Sleep and Dreams

WHY WE SLEEP 6 7 same is true for humans. lndividuals wh0 are deliberately fasting will sleep less as the brain is tricked into thinking that food has suddenly become scarce. Another rare example is the jOint sleep deprivation that occurs in female killer whales and their newborn calves. Female killer whales give birth to a single calf once every three to eight years. Calving normally takes place away 仕 om the other members of the pod. This leaves the newborn calfincredibly vulnerable during the initial weeks oflife, espe- cially during the return tO the pod as it swims beside its mother. Up tO 50 percent of all new calves are killed during this journey home. lt is so dangerous, in fact, that neither mother nor calf appear t0 sleep while in transit. NO mother-calf pair that scientists have observed shows signs of robust sleep en route. This is especially surprising ⅲ the calf, since the highest demand and consumption ofsleep in every other living spe- cies is in the first days and weeks oflife, as any new parent will tell you. Such is the egreglous peril of long-range ocean travel that these infant whales will reverse an otherwise universal sleep trend. Yet the most incredible feat of deliberate sleep deprivation belongs tO that Of birds during transoceanic migration. During this climate- driven race across thousands Of miles, entire flocks will fly for many more hours than is normal. AS a result, they lose much Of the station- ary opportunity for plentiful sleep. But even here, the brain has found an ingenious way t0 obtain sleep. ln-flight, nugrating birds will grab remarkablybriefperiods ofsleep lasting only seconds ⅲ duration. These ultra—power naps are just sufficient tO avert the ruinous brain and bOdy deficits that would otherwise ensue from prolonged total sleep depriva- tion. (lfyou're wondering, humans have no such similar ability.) The white-crowned sparrow is perhaps the most astonishing exam- ple 0f avian sleep deprivation during long-distance flights. This small, quotidian bird is capable 0f a spectacular feat that the American mili- tary has spent millions ofresearch dollars studying. The sparrow has an unparalleled, though time-limited, resilience t0 total sleep deprivation, one that we humans could never withstand. If you sleep-deprive this sparrow in the laboratory during the migratoryperiod ofthe year (when it would otherwise be ⅲ flight), it suffers virtually no ill effects whatso- ever. However, depriving the same sparrow Ofthe same amount ofsleep

10. Why We Sleep The New Science of Sleep and Dreams

CHAPTER 11 Dream Creativity and Dream ControI Aside 仕 om being a stoic sentinel that guards your sanity and emotional well-being, REM sleep and the act of dreaming have another distinct benefit: intelligent information processing that inspires creativity and promotes problem solving. SO much so, that some individuals try con- trolling this normally non-volitional process and direct their own dream expenences While dreaming. DREAMING: THE CREATIVE INCUBATOR Deep NREM sleep strengthens individual memones, as we now know. But it is REM sleep that offers the masterful and complementary benefit of fusing and blending those elemental ingredients together, in abstract and highly novel ways. During the dreaming sleep state, your brain ⅶⅡ cogitate vast swaths ofacquired knowledge,* and then extract overarch- ing rules and commonalities—"the gist." We awake with a revised "Mind Wide Web" that is c 叩 able ofdivining solutions to previously impenetra- ble problems. ln this way, REM-sleep dreaming is informational alchemy. From this dreaming process, which I would describe as ideasthe- sia, have come some ofthe most revolutionary leaps forward in human progress. There is perhaps no better illustration highlighting the smarts *One example is language learning, and the extraction of new grammatical rules. ChiIdren exemplify this. They will start using the laws Of grammar (). g. , conjunctions, tenses, pro- nouns, etc. ) long before they understand what these things are. lt is during sleep that their brains implicitly extract these rules, based on waking experience, despite the child lacking explicit awareness Ofthe rules.