## Metadata - Author: [[Matthew Walker]] - Full Title: Why We Sleep - Category: #books ## Highlights - 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 animal kingdom—the drive to sleep—has continued to elude science for millennia. ([Location 65](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=65)) - every animal species carefully studied to date sleeps.III This suggests 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. ([Location 75](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=75)) - 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). ([Location 83](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=83)) - 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. ([Location 88](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=88)) - every species studied to date sleeps.IV 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. ([Location 98](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=98)) - Everyone generates a circadian rhythm (circa, meaning “around,” and dian, derivative of diam, meaning “day”). Indeed, most living creatures on the planet with a life span of more than several days generate this natural cycle. The internal twenty-four-hour clock within your brain communicates its daily circadian rhythm signal to every other region of your brain and every organ in your body. ([Location 185](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=185)) - Everyone generates a circadian rhythm (circa, meaning “around,” and dian, derivative of diam, meaning “day”). Indeed, every living creature on the planet with a life span of more than several days generates this natural cycle. The internal twenty-four-hour clock within your brain communicates its daily circadian rhythm signal to every other region of your brain and every organ in your body. ([Location 206](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=206)) - 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. Indeed, the reason most living species likely adopted a circadian rhythm is to synchronize themselves and their activities, both internal (e.g., temperature) and external (e.g., feeding), with the daily orbital mechanics of planet Earth spinning on its axis, resulting in regular phases of light (sun facing) and dark (sun hiding). ([Location 249](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=249)) - Yet daylight isn’t the only signal that the brain can latch on to for the purpose of biological clock resetting, though it is the principal and preferential signal, when present. So long as they are reliably repeating, the brain can also use other external cues, such as food, exercise, temperature fluctuations, and even regularly timed social interaction. All of these events have the ability to reset the biological clock, allowing it to strike a precise twenty-four-hour note. ([Location 253](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=253)) - Any signal that the brain uses for the purpose of clock resetting is termed a zeitgeber, from the German “time giver” or “synchronizer.” ([Location 258](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=258)) - You may have noticed that it feels harder to acclimate to a new time zone when traveling eastward than when flying westward. There are two reasons for this. First, the eastward direction requires that you fall asleep earlier than you would normally, which is a tall biological order for the mind to simply will into action. In contrast, the westward direction requires you to stay up later, which is a consciously and pragmatically easier prospect. Second, you will remember that when shut off from any outside world influences, our natural circadian rhythm is innately longer than one day—about twenty-four hours and fifteen minutes. Modest as this may be, this makes it somewhat easier for you to artificially stretch a day than shrink it. When you travel westward—in the direction of your innately longer internal clock—that “day” is longer than twenty-four hours for you and why it feels a little easier to accommodate to. Eastward travel, however, which involves a “day” that is shorter in length for you than twenty-four hours, goes against the grain of your innately long internal rhythm to start with, which is why it is rather harder to do. ([Location 394](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=394)) - a key function of deep NREM sleep, which predominates early in the night, is to do the work of weeding out and removing unnecessary neural connections. In contrast, the dreaming stage of REM sleep, which prevails later in the night, plays a role in strengthening those connections. ([Location 688](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=688)) - Let’s say that you go to bed this evening at midnight. But instead of waking up at eight a.m., getting a full eight hours of sleep, you must wake up at six a.m. because of an early-morning meeting or because you are an athlete whose coach demands early-morning practices. What percent of sleep will you lose? The logical answer is 25 percent, since waking up at six a.m. will lop off two hours of sleep from what would otherwise be a normal eight hours. But that’s not entirely true. Since your brain desires most of its REM sleep in the last part of the night, which is to say the late-morning hours, you may lose 60 to 90 percent of all your REM sleep, even though you are losing 25 percent of your total sleep time. It works both ways. If you wake up at ten a.m., but don’t go to bed until four a.m., then you will lose a significant amount of your normal deep NREM sleep. ([Location 706](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=706)) - Let’s say that you go to bed this evening at midnight. But instead of waking up at eight a.m., getting a full eight hours of sleep, you must wake up at six a.m. because of an early-morning meeting or because you are an athlete whose coach demands early-morning practices. What percent of sleep will you lose? The logical answer is 25 percent, since waking up at six a.m. will lop off two hours of sleep from what would otherwise be a normal eight hours. But that’s not entirely true. Since your brain desires most of its REM sleep in the last part of the night, which is to say the late-morning hours, you will lose 60 to 90 percent of all your REM sleep, even though you are losing 25 percent of your total sleep time. It works both ways. If you wake up at eight a.m., but don’t go to bed until two a.m., then you lose a significant amount of deep NREM sleep. ([Location 722](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=722)) - While AM radio waves cannot match the muscular, dynamic quality of FM radio, the pedestrian pace of AM radio waves gives them the ability to cover vast distances with less fade. Longer-range broadcasts are therefore possible with the slow waves of AM radio, allowing far-reaching communication between very distant geographic locations. As your brain shifts from the fast-frequency activity of waking to the slower, more measured pattern of deep NREM sleep, the very same long-range communication advantage becomes possible. The steady, slow, synchronous waves that sweep across the brain during deep sleep open up communication possibilities between distant regions of the brain, allowing them to collaboratively send and receive their different repositories of stored experience. In this regard, you can think of each individual slow wave of NREM sleep as a courier, able to carry packets of information between different anatomical brain centers. One benefit of these traveling deep-sleep brainwaves is a file-transfer process. Each night, the long-range brainwaves of deep sleep will move memory packets (recent experiences) from a short-term storage site, which is fragile, to a more permanent, and thus safer, long-term storage location. ([Location 798](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=798)) - When it comes to information processing, think of the wake state principally as reception (experiencing and constantly learning the world around you), NREM sleep as reflection (storing and strengthening those raw ingredients of new facts and skills), and REM sleep as integration (interconnecting these raw ingredients with each other, with all past experiences, and, in doing so, building an ever more accurate model of how the world works, including innovative insights and problem-solving abilities). ([Location 825](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=825)) - Why did evolution decide to outlaw muscle activity during REM sleep? Because by eliminating muscle activity you are prevented from acting out your dream experience. ([Location 844](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=844)) - It is perhaps unsurprising that in the small enclaves of Greece where siestas still remain intact, such as the island of Ikaria, men are nearly four times as likely to reach the age of ninety as American males. These napping communities have sometimes been described as “the places where people forget to die.” ([Location 1134](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1134)) - Alcohol consumed by a mother readily crosses the placental barrier, and therefore readily infuses her developing fetus. Knowing this, scientists first examined the extreme scenario: mothers who were alcoholics or heavy drinkers during pregnancy. Soon after birth, the sleep of these neonates was assessed using electrodes gently placed on the head. The newborns of heavy-drinking mothers spent far less time in the active state of REM sleep compared with infants of similar age but who were born of mothers who did not drink during pregnancy. ([Location 1317](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1317)) - During the fourteen hours of total shut-eye per day that a six-month-old infant obtains, there is a 50/50 timeshare between NREM and REM sleep. A five-year-old, however, will have a 70/30 split between NREM and REM sleep across the eleven hours of total daily slumber. In other words, the proportion of REM sleep decreases in early childhood while the proportion of NREM sleep actually increases, even though total sleep time decreases. The downgrading of the REM-sleep portion, and the upswing in NREM-sleep dominance, continues, throughout early and midchildhood. That balance will finally stabilize to an 80/20 NREM/REM sleep split by the late teen years, and remain so throughout early and midadulthood. ([Location 1377](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1377)) - During mid- and late childhood, Feinberg observed moderate deep-sleep amounts as the last neural growth spurts inside the brain were being completed, analogous to late spring and early summer. Then Feinberg began seeing a sharp rise in deep-sleep intensity in his electrical recordings, right at the time when the developmental needs of brain connectivity switch from growing connections to shedding them; the tree’s equivalent of fall. Just as maturational fall was about to turn to winter, and the shedding was nearly complete, Feinberg’s recordings showed a clear ramping back down in deep NREM-sleep intensity to lower intensity once more. The life cycle of childhood was over, and as the last leaves dropped, the onward neural passage of these teenagers had been secured. Deep NREM sleep had aided their transition into early adulthood. ([Location 1420](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1420)) - Feinberg made a second seminal discovery. When he examined the timeline of changing deep-sleep intensity at each different electrode spot on the head, it was not the same. Instead, the rise-and-fall pattern of maturation always began at the back of the brain, which performs the functions of visual and spatial perception, and then progressed steadily forward as adolescence progressed. Most striking, the very last stop on the maturational journey was the tip of the frontal lobe, which enables rational thinking and critical decision-making. Therefore, the back of the brain of an adolescent was more adult-like, while the front of the brain remained more child-like at any one moment during this developmental window of time. ([Location 1432](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1432)) - Of concern is that administering caffeine to juvenile rats will also disrupt deep NREM sleep and, as a consequence, delay numerous measures of brain maturation and the development of social activity, independent grooming, and the exploration of the environment—measures of self-motivated learning. ([Location 1449](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1449)) - we have also observed that in young individuals who are at high risk of developing schizophrenia, and in teenagers and young adults with schizophrenia, there is a two- to threefold reduction in deep NREM sleep. ([Location 1461](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1461)) - If this remains perplexing to parents, a different way to frame and perhaps appreciate the mismatch is this: asking your teenage son or daughter to go to bed and fall asleep at ten p.m. is the circadian equivalent of asking you, their parent, to go to sleep at seven or eight p.m. No matter how loud you enunciate the order, no matter how much that teenager truly wishes to obey your instruction, and no matter what amount of willed effort is applied by either of the two parties, the circadian rhythm of a teenager will not be miraculously coaxed into a change. Furthermore, asking that same teenager to wake up at seven the next morning and function with intellect, grace, and good mood is the equivalent of asking you, their parent, to do the same at four or five a.m. ([Location 1486](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1486)) - Sadly, neither society nor our parental attitudes are well designed to appreciate or accept that teenagers need more sleep than adults, and that they are biologically wired to obtain that sleep at a different time from their parents. It’s very understandable for parents to feel frustrated in this way, since they believe that their teenager’s sleep patterns reflect a conscious choice and not a biological edict. But non-volitional, non-negotiable, and strongly biological they are. We parents would be wise to accept this fact, and to embrace it, encourage it, and praise it, lest we wish our own children to suffer developmental brain abnormalities or force a raised risk of mental illness upon them. ([Location 1492](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1492)) - Central to the goal of adolescent development is the transition from parental dependence to independence, all the while learning to navigate the complexities of peer-group relationships and interactions. One way in which Mother Nature has perhaps helped adolescents unbuckle themselves from their parents is to march their circadian rhythms forward in time, past that of their adult mothers and fathers. This ingenious biological solution selectively shifts teenagers to a later phase when they can, for several hours, operate independently—and do so as a peer-group collective. It is not a permanent or full dislocation from parental care, but as safe an attempt at partially separating soon-to-be adults from the eyes of Mother and Father. There is risk, of course. But the transition must happen. And the time of day when those independent adolescent wings unfold, and the first solo flights from the parental nest occur, is not a time of day at all, but rather a time of night, thanks to a forward-shifted circadian rhythm. ([Location 1503](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1503)) - As you enter your fourth decade of life, there is a palpable reduction in the electrical quantity and quality of that deep NREM sleep. You obtain fewer hours of deep sleep, and those deep NREM brainwaves become smaller, less powerful, and fewer in number. Passing into your mid- and late forties, age will have stripped you of 60 to 70 percent of the deep sleep you were enjoying as a young teenager. By the time you reach seventy years old, you will have lost 80 to 90 percent of your youthful deep sleep. ([Location 1527](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1527)) - The second hallmark of altered sleep as we age, and one that older adults are more conscious of, is fragmentation. The older we get, the more frequently we wake up throughout the night. There are many causes, including interacting medications and diseases, but chief among them is a weakened bladder. Older adults therefore visit the bathroom more frequently at night. Reducing fluid intake in the mid- and late evening can help, but it is not a cure-all. Due to sleep fragmentation, older individuals will suffer a reduction in sleep efficiency, defined as the percent of time you were asleep while in bed. If you spent eight hours in bed, and slept for all eight of those hours, your sleep efficiency would be 100 percent. If you slept just four of those eight hours, your sleep efficiency would be 50 percent. ([Location 1540](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1540)) - As healthy teenagers, we enjoyed a sleep efficiency of about 95 percent. As a reference anchor, most sleep doctors consider good-quality sleep to involve a sleep efficiency of 90 percent or above. By the time we reach our eighties, sleep efficiency has often dropped below 70 or 80 percent; 70 to 80 percent may sound reasonable until you realize that, within an eight-hour period in bed, it means you will spend as much as one to one and a half hours awake. ([Location 1546](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1546)) - The third sleep change with advanced age is that of circadian timing. In sharp contrast to adolescents, seniors commonly experience a regression in sleep timing, leading to earlier and earlier bedtimes. The cause is an earlier evening release and peak of melatonin as we get older, instructing an earlier start time for sleep. Restaurants in retirement communities have long known of this age-related shift in bedtime preference, epitomized (and accommodated) by the “early-bird special.” ([Location 1565](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1565)) - Evening light suppresses the normal rise in melatonin, pushing an average adult’s sleep onset time into the early-morning hours, preventing sleep at a reasonable hour. However, this same sleep-delaying effect can be put to good use in older adults, if timed correctly. Having woken up early, many older adults are physically active during the morning hours, and therefore obtain much of their bright-light exposure in the first half of the day. This is not optimal, as it reinforces the early-to-rise, early-to-decline cycle of the twenty-four-hour internal clock. Instead, older adults who want to shift their bedtimes to a later hour should get bright-light exposure in the late-afternoon hours. I am not, however, suggesting that older adults stop exercising in the morning. Exercise can help solidify good sleep, especially in the elderly. Instead, I advise two modifications for seniors. First, wear sunglasses during morning exercise outdoors. This will reduce the influence of morning light being sent to your suprachiasmatic clock that would otherwise keep you on an early-to-rise schedule. Second, go back outside in the late afternoon for sunlight exposure, but this time do not wear sunglasses. Make sure to wear sun protection of some sort, such as a hat, but leave the sunglasses at home. Plentiful later-afternoon daylight will help delay the evening release of melatonin, helping push the timing of sleep to a later hour. Older adults may also wish to consult with their doctor about taking melatonin in the evening. Unlike young or middle-age adults, where melatonin has not proved efficacious for helping sleep beyond the circumstance of jet lag, prescription melatonin has been shown to help boost the otherwise blunted circadian and associated melatonin rhythm in the elderly, reducing the time taken to fall asleep and improving self-reported sleep quality and morning alertness. ([Location 1587](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1587)) - What we have at least discovered is a rather telling pattern of looping activity in the brain that coincides with these speedy sleep spindles. The activity circles between the memory storage site (the hippocampus) and those regions that program the decision of intentionality (in the frontal lobe), such as “This is important” or “This is irrelevant.” The recursive cycle of activity between these two areas (memory and intentionality), which happens ten to fifteen times per second during the spindles, may help explain NREM sleep’s discerning memory influence. Much like selecting intentional filters on an Internet search or a shopping app, spindles offer a refining benefit to memory by allowing the storage site of your hippocampus to check in with the intentional filters carried in your astute frontal lobes, allowing selection only of that which you need to save, while discarding that which you do not. ([Location 1973](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=1973)) - Those who remained awake across the day showed no evidence of a significant improvement in performance. However, fitting with the pianist’s original description, those who were tested after the very same time delay of twelve hours, but that spanned a night of sleep, showed a striking 20 percent jump in performance speed and a near 35 percent improvement in accuracy. Importantly, those participants who learned the motor skill in the morning—and who showed no improvement that evening—did go on to show an identical bump up in performance when retested after a further twelve hours, now after they, too, had had a full night’s sleep. ([Location 2028](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2028)) - Sleep had again transferred the memories, but the results were different from that for textbook-like memory. Rather than a transfer from short- to long-term memory required for saving facts, the motor memories had been shifted over to brain circuits that operate below the level of consciousness. As a result, those skill actions were now instinctual habits. ([Location 2051](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2051)) - The increases in speed and accuracy, underpinned by efficient automaticity, were directly related to the amount of stage 2 NREM, especially in the last two hours of an eight-hour night of sleep (e.g., from five to seven a.m., should you have fallen asleep at eleven p.m.). ([Location 2057](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2057)) - in 2015, the International Olympic Committee published a consensus statement highlighting the critical importance of, and essential need for, sleep in athletic development across all sports for men and women. ([Location 2079](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2079)) - It is of little surprise that we see a spike in stage 2 NREM sleep, including sleep spindles, right around the infant’s time of transition from crawling to walking. ([Location 2115](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2115)) - Struck by the weight of damning scientific evidence, the Guinness Book of World Records has stopped recognizing attempts to break the sleep deprivation world record. Recall that Guinness deems it acceptable for a man (Felix Baumgartner) to ascend 128,000 feet into the outer reaches of our atmosphere in a hot-air balloon wearing a spacesuit, open the door of his capsule, stand atop a ladder suspended above the planet, and then free-fall back down to Earth at a top speed of 843 mph (1,358 kmh), passing through the sound barrier while creating a sonic boom with just his body. But the risks associated with sleep deprivation are considered to be far, far higher. ([Location 2167](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2167)) - you don’t need to fall asleep for ten to fifteen seconds to die while driving. Two seconds will do it. A two-second microsleep at 30 mph with a modest angle of drift can result in your vehicle transitioning entirely from one lane to the next. ([Location 2190](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2190)) - Drowsy driving alone is worse than driving drunk. That may seem like a controversial or irresponsible thing to say, and I do not wish to trivialize the lamentable act of drunk driving by any means. Yet my statement is true for the following simple reason: drunk drivers are often late in braking, and late in making evasive maneuvers. But when you fall asleep, or have a microsleep, you stop reacting altogether. ([Location 2195](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2195)) - Most worrying from a societal perspective were the individuals in the group who obtained six hours of sleep a night—something that may sound familiar to many of you. Ten days of six hours of sleep a night was all it took to become as impaired in performance as going without sleep for twenty-four hours straight. And like the total sleep deprivation group, the accruing performance impairment in the four-hour and six-hour sleep groups showed no signs of leveling out. All signs suggested that if the experiment had continued, the performance deterioration would continue to build up over weeks or months. ([Location 2226](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2226)) - The third key finding, common to both of these studies, is the one I personally think is the most harmful of all. When participants were asked about their subjective sense of how impaired they were, they consistently underestimated their degree of performance disability. It was a miserable predictor of how bad their performance actually, objectively was. ([Location 2233](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2233)) - Similarly problematic is baseline resetting. With chronic sleep restriction over months or years, an individual will actually acclimate to their impaired performance, lower alertness, and reduced energy levels. That low-level exhaustion becomes their accepted norm, or baseline. ([Location 2237](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2237)) - Sixty years of scientific research prevent me from accepting anyone who tells me that he or she can “get by on just four or five hours of sleep a night just fine.” ([Location 2243](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2243)) - After being awake for nineteen hours, people who were sleep-deprived were as cognitively impaired as those who were legally drunk. Said another way, if you wake up at seven a.m. and remain awake throughout the day, then go out socializing with friends until late that evening, yet drink no alcohol whatsoever, by the time you are driving home at two a.m. you are as cognitively impaired in your ability to attend to the road and what is around you as a legally drunk driver. In fact, participants in the above study started their nosedive in performance after just fifteen hours of being awake (ten p.m. in the above scenario). ([Location 2251](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2251)) - Those in the eight-hour sleep group had few, if any, off-road errors. Those in the four-hour sleep condition (the second group) had six times more off-road deviations than the sober, well-rested individuals. The same degree of driving impairment was true of the third group, who had eight hours of sleep but were legally drunk. Driving drunk or driving drowsy were both dangerous, and equally dangerous. A reasonable expectation was that performance in the fourth group of participants would reflect the additive impact of these two groups: four hours of sleep plus the effect of alcohol (i.e., twelve times more off-road deviations). It was far worse. This group of participants drove off the road almost thirty times more than the well-rested, sober group. The heady cocktail of sleep loss and alcohol was not additive, but instead multiplicative. They magnified each other, like two drugs whose effects are harmful by themselves but, when taken together, interact to produce truly dire consequences. ([Location 2270](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2270)) - drunk drivers are often late in braking, and late in making evasive maneuvers. But when you fall asleep, or have a microsleep, you stop reacting altogether. ([Location 2286](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2286)) - When Dinges and Rosekind reported their findings to the FAA, they recommended that “prophylactic naps”—naps taken early during long-haul flights—should be instituted as policy among pilots, as many other aviation authorities around the world now permit. The FAA, while believing the findings, was not convinced by the nomenclature. They believed the term “prophylactic” was ripe for many a snide joke among pilots. Dinges suggested the alternative of “planned napping.” The FAA didn’t like this, either, feeling it to be too “management-like.” Their suggestion was “power napping,” which they believed was more fitting with leadership- or dominance-based job positions, others being CEOs or military executives. And so the “power nap” was born. ([Location 2343](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2343)) - To impress this fact further, I quote one of my research colleagues, Dr. Thomas Roth at the Henry Ford Hospital in Detroit, who once said, “The number of people who can survive on five hours of sleep or less without any impairment, expressed as a percent of the population, and rounded to a whole number, is zero.” ([Location 2366](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2366)) - A structure located in the left and right sides of the brain, called the amygdala—a key hot spot for triggering strong emotions such as anger and rage, and linked to the fight-or-flight response—showed well over a 60 percent amplification in emotional reactivity in the participants who were sleep-deprived. In contrast, the brain scans of those individuals who were given a full night’s sleep evinced a controlled, modest degree of reactivity in the amygdala, despite viewing the very same images. It was as though, without sleep, our brain reverts to a primitive pattern of uncontrolled reactivity. We produce unmetered, inappropriate emotional reactions, and are unable to place events into a broader or considered context. ([Location 2385](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2385)) - Why were the emotion centers of the brain so excessively reactive without sleep? Further MRI studies using more refined analyses allowed us to identify the root cause. After a full night of sleep, the prefrontal cortex—the region of the brain that sits just above your eyeballs; is most developed in humans, relative to other primates; and is associated with rational, logical thought and decision-making—was strongly coupled to the amygdala, regulating this deep emotional brain center with inhibitory control. With a full night of plentiful sleep, we have a balanced mix between our emotional gas pedal (amygdala) and brake (prefrontal cortex). Without sleep, however, the strong coupling between these two brain regions is lost. We cannot rein in our atavistic impulses—too much emotional gas pedal (amygdala) and not enough regulatory brake (prefrontal cortex). Without the rational control given to us each night by sleep, we’re not on a neurological—and hence emotional—even keel. ([Location 2390](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2390)) - We discovered that different deep emotional centers in the brain just above and behind the amygdala, called the striatum—associated with impulsivity and reward, and bathed by the chemical dopamine—had become hyperactive in sleep-deprived individuals in response to the rewarding, pleasurable experiences. As with the amygdala, the heightened sensitivity of these hedonic regions was linked to a loss of the rational control from the prefrontal cortex. ([Location 2405](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2405)) - Sleep disturbance is a recognized hallmark associated with addictive substance use.IV Insufficient sleep also determines relapse rates in numerous addiction disorders, associated with reward cravings that are unmetered, lacking control from the rational head office of the brain’s prefrontal cortex.V Relevant from a prevention standpoint, insufficient sleep during childhood significantly predicts early onset of drug and alcohol use in that same child during their later adolescent years, even when controlling for other high-risk traits, such as anxiety, attention deficits, and parental history of drug use. ([Location 2420](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2420)) - There is no major psychiatric condition in which sleep is normal. This is true of depression, anxiety, post-traumatic stress disorder (PTSD), schizophrenia, and bipolar disorder (once known as manic depression). ([Location 2429](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2429)) - Should you improve sleep quality in patients suffering from several psychiatric conditions using a technique we will discuss later, called cognitive behavioral therapy for insomnia (CBT-I), you can improve symptom severity and remission rates. My colleague at the University of California, Berkeley, Dr. Allison Harvey has been a pioneer in this regard. By improving sleep quantity, quality, and regularity, Harvey and her team have systematically demonstrated the healing abilities of sleep for the minds of numerous psychiatric populations. She has intervened with the therapeutic tool of sleep in conditions as diverse as depression, bipolar disorder, anxiety, and suicide, all to great effect. By regularizing and enhancing sleep, Harvey has stepped these patients back from the edge of crippling mental illness. ([Location 2452](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2452)) - “The best bridge between despair and hope is a good night’s sleep.” ([Location 2481](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2481)) - Glial cells are distributed throughout your entire brain, situated side by side with the neurons that generate the electrical impulses of your brain. Just as the lymphatic system drains contaminants from your body, the glymphatic system collects and removes dangerous metabolic contaminants generated by the hard work performed by neurons in your brain, rather like a support team surrounding an elite athlete. Although the glymphatic system—the support team—is somewhat active during the day, Nedergaard and her team discovered that it is during sleep that this neural sanitization work kicks into high gear. Associated with the pulsing rhythm of deep NREM sleep comes a ten- to twentyfold increase in effluent expulsion from the brain. In what can be described as a nighttime power cleanse, the purifying work of the glymphatic system is accomplished by cerebrospinal fluid that bathes the brain. ([Location 2613](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2613)) - Nedergaard made a second astonishing discovery, which explained why the cerebrospinal fluid is so effective in flushing out metabolic debris at night. The glial cells of the brain were shrinking in size by up to 60 percent during NREM sleep, enlarging the space around the neurons and allowing the cerebrospinal fluid to proficiently clean out the metabolic refuse left by the day’s neural activity. ([Location 2620](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2620)) - Nedergaard’s findings completed the circle of knowledge that our findings had left unanswered. Inadequate sleep and the pathology of Alzheimer’s disease interact in a vicious cycle. Without sufficient sleep, amyloid plaques build up in the brain, especially in deep-sleep-generating regions, attacking and degrading them. The loss of deep NREM sleep caused by this assault therefore lessens the ability to remove amyloid from the brain at night, resulting in greater amyloid deposition. More amyloid, less deep sleep, less deep sleep, more amyloid, and so on and so forth. ([Location 2632](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2632)) - When communicating science to the general public in lectures or writing, I’m always wary of bombarding an audience with never-ending mortality and morbidity statistics, lest they themselves lose the will to live in front of me. It is hard not to do so with such compelling masses of studies in the field of sleep deprivation. Often, however, a single astonishing result is all that people need to apprehend the point. For cardiovascular health, I believe that finding comes from a “global experiment” in which 1.5 billion people are forced to reduce their sleep by one hour or less for a single night each year. It is very likely that you have been part of this experiment, otherwise known as daylight savings time. In the Northern Hemisphere, the switch to daylight savings time in March results in most people losing an hour of sleep opportunity. Should you tabulate millions of daily hospital records, as researchers have done, you discover that this seemingly trivial sleep reduction comes with a frightening spike in heart attacks the following day. Impressively, it works both ways. In the autumn within the Northern Hemisphere, when the clocks move forward and we gain an hour of sleep opportunity time, rates of heart attacks plummet the day after. A similar rise-and-fall relationship can be seen with the number of traffic accidents, proving that the brain, by way of attention lapses and microsleeps, is just as sensitive as the heart to very small perturbations of sleep. Most people think nothing of losing an hour of sleep for a single night, believing it to be trivial and inconsequential. It is anything but. ([Location 2671](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2671)) - Adults forty-five years or older who sleep fewer than six hours a night are 200 percent more likely to have a heart attack or stroke during their lifetime, as compared with those sleeping seven to eight hours a night. This finding impresses how important it is to prioritize sleep in midlife—which is unfortunately the time when family and professional circumstances encourage us to do the exact opposite. ([Location 2705](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2705)) - When ([Location 2763](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=2763)) - REM sleep can therefore be considered as a state characterized by strong activation in visual, motor, emotional, and autobiographical memory regions of the brain, yet a relative deactivation in regions that control rational thought. ([Location 3173](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=3173)) - “I know exactly what your dream is about, Kyle.” Amazed, he (and the rest of the lecture hall) awaits, my answer as though time has ground to a halt. After another long pause, I confidently enunciate the following: “Your dream, Kyle, is about time, and more specifically, about not having enough time to do the things you really want to do in life.” A wave of recognition, almost relief, washes over Kyle’s face, and the rest of the class appear equally convinced. Then I come clean. “Kyle—I have a confession. No matter what dream anyone ever tells me, I always give them that very same generic response, and it always seems to fit.” ([Location 3271](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=3271)) - At the heart of the theory was an astonishing change in the chemical cocktail of your brain that takes place during REM sleep. Concentrations of a key stress-related chemical called noradrenaline are completely shut off within your brain when you enter this dreaming sleep state. In fact, REM sleep is the only time during the twenty-four-hour period when your brain is completely devoid of this anxiety-triggering molecule. Noradrenaline, also known as norepinephrine, is the brain equivalent to a body chemical you already know and have felt the effects of: adrenaline ([Location 3349](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=3349)) - Cartwright demonstrated that it was only those patients who were expressly dreaming about the painful experiences around the time of the events who went on to gain clinical resolution from their despair, mentally recovering a year later as clinically determined by having no identifiable depression. ([Location 3407](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=3407)) - Something has to switch the activating influence of the brain stem off, and in doing so, allow sleep to be switched on. That switch—the sleep-wake switch—is located just below the thalamus in the center of the brain, in a region called the hypothalamus. It is the same neighborhood that houses the twenty-four-hour master biological clock, perhaps unsurprisingly. The sleep-wake switch within the hypothalamus has a direct line of communication to the power station regions of the brain stem. Like an electrical light switch, it can flip the power on (wake) or off (sleep). To do this, the sleep-wake switch in the hypothalamus releases a neurotransmitter called orexin. You can think of orexin as the chemical finger that flips the switch to the “on,” wakefulness, position. When orexin is released down onto your brain stem, the switch has been unambiguously flipped, powering up the wakefulness-generating centers of the brain stem. Once activated by the switch, the brain stem pushes open the sensory gate of the thalamus, allowing the perceptual world to flood into your brain, transitioning you to full, stable wakefulness. At night, the opposite happens. The sleep-wake switch stops releasing orexin onto the brain stem. The chemical finger has now flipped the switch to the “off” position, shutting down the rousing influence from the power station of the brain stem. The sensory business being conducted within the thalamus is closed down by a sealing of the sensory gate. We lose perceptual contact with the outside world, and now sleep. Lights off, lights on, lights off, lights on—this is the neurobiological job of the sleep-wake switch in the hypothalamus, controlled by orexin. ([Location 4019](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=4019)) - Scientists have examined the brains of narcoleptic patients in painstaking detail after they have passed away. During these postmortem investigations, they discovered a loss of almost 90 percent of all the cells that produce orexin. Worse still, the welcome sites, or receptors, of orexin that cover the surface of the power station of the brain stem were significantly reduced in number in narcoleptic patients, relative to normal individuals. ([Location 4035](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=4035)) - alcohol is one of the most powerful suppressors of REM sleep that we know of. When the body metabolizes alcohol it produces by-product chemicals called aldehydes and ketones. The aldehydes in particular will block the brain’s ability to generate REM sleep. It’s rather like the cerebral version of cardiac arrest, preventing the pulsating beat of brainwaves that otherwise power dream sleep. People consuming even moderate amounts of alcohol in the afternoon and/or evening are thus depriving themselves of dream sleep. ([Location 4248](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=4248)) - Alcohol is in a class of drugs called sedatives. It binds to receptors within the brain that prevent neurons from firing their electrical impulses. Saying that alcohol is a sedative often confuses people, as alcohol in moderate doses helps individuals liven up and become more social. How can a sedative enliven you? The answer comes down to the fact that your increased sociability is caused by sedation of one part of your brain, the prefrontal cortex, early in the timeline of alcohol’s creeping effects. As we have discussed, this frontal lobe region of the human brain helps control our impulses and restrains our behavior. Alcohol immobilizes that part of our brain first. As a result, we “loosen up,” becoming less controlled and more extroverted. But anatomically targeted brain sedation it still is. Give alcohol a little more time, and it begins to sedate other parts of the brain, dragging them down into a stupefied state, just like the prefrontal cortex. You begin to feel sluggish as the inebriated torpor sets in. This is your brain slipping into sedation. Your desire and ability to remain conscious are decreasing, and you can let go of consciousness more easily. I am very deliberately avoiding the term “sleep,” however, because sedation is not sleep. Alcohol sedates you out of wakefulness, but it does not induce natural sleep. The electrical brainwave state you enter via alcohol is not that of natural sleep; rather, it is akin to a light form of anesthesia. ([Location 4341](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=4341)) - alcohol fragments sleep, littering the night with brief awakenings. Alcohol-infused sleep is therefore not continuous and, as a result, not restorative. Unfortunately, most of these nighttime awakenings go unnoticed by the sleeper since they don’t remember them. Individuals therefore fail to link alcohol consumption the night before with feelings of next-day exhaustion caused by the undetected sleep disruption sandwiched in between. Keep an eye out for that coincidental relationship in yourself and/or others. ([Location 4354](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=4354)) - alcohol will often suppress REM sleep, especially during the first half or two-thirds of the night. When the body metabolizes alcohol it produces by-product chemicals called aldehydes and ketones. The aldehydes in particular will block the brain’s ability to generate REM sleep. It’s rather like the cerebral version of cardiac arrest, preventing the pulsating beat of brainwaves that otherwise power dream sleep. People consuming even moderate amounts of alcohol in the afternoon and/or evening can inadvertently deprive themselves of dream sleep. ([Location 4357](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=4357)) - The obvious methods involve reducing caffeine and alcohol intake, removing screen technology from the bedroom, and having a cool bedroom. In addition, patients must (1) establish a regular bedtime and wake-up time, even on weekends, (2) go to bed only when sleepy and avoid sleeping on the couch early/mid-evenings, (3) never lie awake in bed for a significant time period; rather, get out of bed and do something quiet and relaxing until the urge to sleep returns, (4) avoid daytime napping if you are having difficulty sleeping at night, (5) reduce anxiety-provoking thoughts and worries by learning to mentally decelerate before bed, and (6) remove visible clockfaces from view in the bedroom, preventing clock-watching anxiety at night. ([Location 4669](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=4669)) - What you eat also appears to have some impact on your nighttime sleep. Eating a high-carbohydrate, low-fat diet for two days decreases the amount of deep NREM sleep at night, but increases the amount of REM sleep dreaming, relative to a two-day diet low in carbohydrates and high in fat. In a carefully controlled study of healthy adult individuals, a four-day diet high in sugar and other carbohydrates, but low in fiber, resulted in less deep NREM sleep and more awakenings at night.XI ([Location 4732](https://readwise.io/to_kindle?action=open&asin=B06ZZ1YGJ5&location=4732))