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RokNewsPager Deep into Sleep 深度睡眠

Deep into Sleep 深度睡眠

缺乏睡眠损害性能在各种各样的任务。一个单一通宵三重反应时间,大大增加注意的失误。 不久前, 在私人执业精神科医生打电话,精神病学副教授,专门从事睡眠研究的认知神经学家罗伯特Stickgold。他询问是否Stickgold的任何理由不订明莫达非尼,一个新的促进觉醒的药物,很多在考试期间的学术工作面临的一个哈佛大学本科知道。

不 的东西。“ 当人们不可能声称,他们每晚睡眠4小时,Stickgold常常问他们是否担心他们正在失去什么。 “你得到了一个空白的样子,”他说。 “他们认为睡眠是浪费时间。”,但睡不只是“时间”活着的发作之间。在一个渐进的框架内,一个简单的事实,我们对我们生活的三分之一花费睡着了,睡眠是必要之恶多。大部分蒸发,而我们都睡着了,问题已经不再是 是否 睡眠的东西,但确切的。睡眠不足可能与肥胖,糖尿病,免疫系统功能障碍,和许多疾病,以及安全问题,如汽车事故和医疗差错,加上受损的工作表现和许多其他活动的生产率。 虽然现代睡眠研究的时代在20世纪50年代开始的REM(快速眼动)睡眠的发现,该领域仍然存在,直到最近,昏昏欲睡。甚至20年前,“睡眠研究的主导范式是”睡眠疗法嗜睡“,”说Stickgold。此后,研究人员已经开发出一种更为复杂的画面会发生什么,而我们贪睡。在睡眠医学的年度会议,今年才成为公认的医学专科,现在提请5000参与者。哈佛大学一直在该地区的领导者。医学院的睡眠医学部,成立于1997年和睡眠医学教授查尔斯Czeisler Baldino主持,有61个院系分支机构。该部门旨在促进睡眠,睡眠障碍,昼夜生物学合作研究,教育医生和普通公众,影响公共政策,并设置新标准的临床实践,针对其网站,(www.hms.harvard.edu/sleep) 声明,创建“在睡眠和昼夜生物学模型方案。”  

“睡眠贪食”文化

试想上露营之旅没有手电筒或灯笼。随着太阳在这一天结束套,白天逐渐让位黑暗,一旦营火烧毁,你可能会进入睡眠状态。日出,有一个类似的反向的梯度,从刚开始的时候,人类一直夹带光明与黑暗的这些周期。 智人 是不是夜行性动物,我们没有良好的夜视并没有在黑暗中特别有效。然而,在瞬间的进化时间规模上,爱迪生的发明的灯泡,和他的第一轮时钟珍珠在曼哈顿街的权力在1882年工厂开幕,转移方向夜间时间,和,光的环境的。在一个开关单元,夜间活动的整个范围,开辟了,今天我们生活在一个24小时的世界,始终是工作或游玩。电视和电话从来没有关闭,互联网,让您购物,赌博,工作,或在凌晨3点调情;企业保持开放的不断更长的时间;几十每年以百万计的旅客跨多个时区,全球增长全球商业和通信,华尔街交易员可能需要早起或熬夜,以保持日本的日经交易所或在德意志联邦银行的发展。 因此大部分我们现在睡眠少的人比一个世纪前,甚至50年前。 2005年全国睡眠基金会的民意调查显示,平均6.8小时的睡眠时间上周日的夜间不到一个小时不到他们需要更多的成年美国人,Czeisler说。不仅是如何 睡眠,但  people sleep has changed. In the United States, six to eight million shift workers toil regularly at night, disrupting sleep patterns in ways that are not necessarily amenable to adaptation. Many people get only five hours per night during the week and then try to catch up by logging nine hours nightly on weekends. “You can make up for acute sleep deprivation,” says David P. White, McGinness professor of sleep medicine and director of the sleep disorders program at Brigham and Women’s Hospital. “But we don’t know what happens when people are chronically sleep-deprived over years.” “We are living in the middle of history’s greatest experiment in sleep deprivation and we are all a part of that experiment,” says Stickgold. “It’s not inconceivable to me that we will discover that there are major social, economic, and health consequences to that experiment. Sleep deprivation doesn’t have any good side effects.” All animals sleep. Fish that need to keep swimming to breathe sleep with half their brains while the other half keeps them moving. It is uncertain whether fruit flies actually sleep (“We can’t put electrodes in their brains,” says White), but they seem at least to rest, because for extended periods they do not move. When researchers stopped fruit flies from resting by swatting at them, the flies took even longer rest periods. When lab technicians added caffeine to the water that the flies drank, they stayed active longer—and also rested longer after the drug wore off, evidence that the caffeine had disrupted their resting patterns. Sleeping well helps keep you alive longer. Among humans, death from all causes is lowest among adults who get seven to eight hours of sleep nightly, and significantly higher among those who sleep less than seven or more than nine hours. (“Those who sleep more than nine hours have something wrong with them that may be causing the heavy sleep, and leads to their demise,” White notes. “It is not the sleep itself that is harmful.”) Sleep is essential to normal biological function. “The immune system doesn’t work well if we don’t sleep,” says White. “Most think sleep serves some neurological process to maintain homeostasis in the brain.” Rats totally deprived of sleep die in 17 to 20 days: their hair starts falling out, and they become hypermetabolic, burning lots of calories while just standing still. There once was a fair amount of research on total sleep deprivation, like that which killed the rats. Doctors would keep humans awake for 48, 72, or even 96 hours, and watch their performance deteriorate while their mental states devolved into psychosis. For several reasons, such studies rarely happen any more (“Why study something that doesn’t exist?” asks White) and researchers now concentrate on sleep restriction studies. In this context, it is important to distinguish between acute and chronic sleep deprivation. Someone who misses an entire night of sleep but then gets adequate sleep on the following three days “will recover most of his or her normal ability to function, ” Czeisler says. “But someone restricted to only five hours of nightly sleep for weeks builds up a cumulative sleep deficit. In the first place, their performance will be as impaired as if they had been up all night. Secondly, it will take two to three weeks of extra nightly sleep before they return to baseline performance. Chronic sleep deprivation’s impact takes much longer to build up, and it also takes much longer to recover.” The body is eager to restore the balance; Harvard undergraduates, a high-achieving, sleep-deprived population, frequently go home for Christmas vacation and pretty much sleep for the first week. Stickgold notes that “When you live on four hours a night, you forget what it’s like to really be awake.” Sleep researcher Eve van Cauter at the University of Chicago exposed sleep-deprived students (allowed only four hours per night for six nights) to flu vaccine; their immune systems produced only half the normal number of antibodies in response to the viral challenge. Levels of cortisol (a hormone associated with stress) rose, and the sympathetic nervous system became active, raising heart rates and blood pressure. The subjects also showed insulin resistance, a pre-diabetic condition that affects glucose tolerance and produces weight gain. “[When] restricted to four hours [of sleep] a night, within a couple of weeks, you could make an 18-year-old look like a 60-year-old in terms of their ability to metabolize glucose,” Czeisler notes. “The sleep-deprived metabolic syndrome might increase carbohydrate cravings and the craving for junk food.” Van Cauter also showed that sleep-deprived subjects had reduced levels of leptin, a molecule secreted by fat cells that acts in the brain to inhibit appetite. “During nights of sleep deprivation, you feel that your eating goes wacky,” says Stickgold. “Up at 2 a.m., working on a paper, a steak or pasta is not very attractive. You’ll grab the candy bar instead. It probably has to do with the glucose regulation going off. It could be that a good chunk of our epidemic of obesity is actually an epidemic of sleep deprivation.” Furthermore, “Many children in our society don’t get adequate amounts of sleep,” Czeisler says. “Contrary to what one might expect, it’s common to see irritability and hyperactivity in sleep-deprived children. Is it really surprising that we treat them with wake-promoting drugs like Ritalin?” Schools and athletic programs press children to stay awake longer, and some children may be chronically sleep-deprived. Czeisler once took his daughter to a swim-team practice that ran from eight to nine o’clock at night, and told the coaches that this was too late an hour for children. “They looked at me like I was from another planet,” he recalls. “They said, ‘This is when we can get the pool.’” Stickgold compares sleep deprivation to eating disorders. “Twenty years ago, bulimics probably thought they had the best of all worlds,” he says. “They could eat all they wanted and never gain weight. Now we know that they were and are doing major damage to their bodies and suffering major psychological damage. We live in a world of sleep bulimia, where we binge on weekends and purge during the week.”  

The Fatigue Tax

Lack of sleep impairs performance on a wide variety of tasks. A single all-nighter can triple reaction time and vastly increase lapses of attention. Sleep researcher David Dinges at the University of Pennsylvania studied such lapses using a “psychomotor vigilance task” on pools of subjects who had slept four, six, or eight hours nightly for two weeks. The researchers measured subjects’ speed of reaction to a computer screen where, at random intervals within a defined 10-minute period, the display would begin counting up in milliseconds from 000 to one second. The task was first, to notice that the count had started, and second, to stop it as quickly as possible by hitting a key. It wasn’t so much that the sleep-deprived subjects were slower, but that they had far more total lapses, letting the entire second go by without responding. Those on four hours a night had more lapses than those sleeping six, who in turn had more lapses than subjects sleeping eight hours per night. “The number of lapses went up and up for the whole two weeks,” says David White, “and they hadn’t plateaued at the end of the two-week study!” There’s fairly large individual variation in susceptibility to the cognitive effects of sleep deprivation: in one of Charles Czeisler’s studies, somewhere between a quarter and a third of the subjects who stayed awake all night contributed two-thirds of the lapses of attention. “Some are more resistant to the impact of a single night of sleep loss,” he says. “But they all fall apart after two nights without sleep.” In a sleep-deprived state, says White, “Most of us can perform at a fairly low level. And a lot can run around sleep-deprived without it being obvious. But truck drivers, neurosurgeons, nuclear-plant workers—after six or eight hours, they have to put a second crew on and give them a break.” Very few people are really immune to sleep deprivation: in Dinges’s study, only one of 48 subjects had the same performance after two weeks of four hours’ nightly sleep as on day one. Students often wonder whether to pull an all-nighter before an exam. Will the extra studying time outweigh the exhaustion? Robert Stickgold, who has studied sleep’s role in cognition for the past 10 years, reports that it depends on the exam. “If you are just trying to remember simple facts—listing all the kings of England, say—cramming all night works, ” he explains. “That’s because it’s a different memory system, the declarative memory system. But if you expect to be hit with a question like ‘Relate the French Revolution to the Industrial Revolution,’ where you have to synthesize connections between facts, then missing that night of sleep can be disastrous. Your ability to do critical thinking takes a massive hit—just as with alcohol, you’re knocking out the frontal-cortex functions. “It’s a version of ‘sleeping on a problem,’” Stickgold continues. “If you can’t recall a phone number, you don’t say, ‘Let me sleep on it.’ But if you can’t decide whether to take a better-paying job located halfway across the country—where you have all the information and just have to weigh it—you say, ‘Let me sleep on it.’ You don’t say, ‘Give me 24 hours.’ We realize that it’s not just time; we understand at a gut level that the brain is doing this integration of information as we sleep, all by itself.” Not only mental and emotional clarification, but the improvement of motor skills can occur while asleep. “Suppose you are trying to learn a passage in a Chopin piano étude, and you just can’t get it,” says Stickgold. “You walk away and the next day, the first try, you’ve got it perfectly. We see this with musicians, and with gymnasts. There’s something about learning motor-activity patterns, complex movements: they seem to get better by themselves, overnight.” Stickgold’s colleague Matthew Walker, an instructor in psychiatry, studied a simple motor task: typing the sequence “41324” as rapidly and accurately as possible. After 12 minutes of training, subjects improved their speed by 50 to 60 percent, but then reached a plateau. Those who trained in the morning and came back for another trial the same evening showed no improvement. But those who trained in the evening and returned for a retest the following morning were 15 to 20 percent faster and 30 to 50 percent more accurate. “Twenty percent improvement—what’s that?” asks Stickgold, rhetorically. “Well, it’s taking a four-minute mile down to three minutes and 10 seconds, or raising a five-foot high jump to six feet.”  

Bodily Rituals

So sleep is essential, but exactly why we go to sleep remains a mystery. Professor of psychiatry Robert McCarley, based at the VA Boston Healthcare System, has linked sleep to the brain neurochemical adenosine. Adenosine binds with phosphorus to create adenosine triphosphate (ATP), a substance that cells break down to generate energy. McCarley and colleagues inserted microcatheters into cat brains while keeping the cats awake for up to six hours—a long time for a cat. They found that rising adenosine levels in the basal forebrain put the cat to sleep; then, in the sleeping cat, adenosine levels fall again. In both cats and humans, the basal forebrain includes cells important for wakefulness, and adenosine turns these cells off, triggering sleep. Like cats, when we are awake and active, we burn ATP, which breaks down to adenosine. Over time, adenosine levels build up, causing pressure for sleep. During sleep, many of the body’s cells are less active and hence burn less ATP, so adenosine levels fall again, setting the stage for wakefulness. A drug like caffeine, however, partially blocks adenosine receptors, so the brain doesn’t perceive the actual adenosine level, and we don’t get tired. In a world that values wakefulness and productivity over rest and recovery, caffeine has become, in dollar amounts, the second-largest commodity (after oil) traded in the world. Some consumers require ever-greater jolts—one 24-ounce Starbucks beverage packs a walloping 1,000-plus milligrams of caffeine. (A commonly used figure for one cup of coffee is 100 milligrams.) The lab run by Putnam professor of neurology Clifford Saper has done related research, refining the location and functions of the “sleep switch,” a group of nerve cells in the hypothalamus that turns off the brain’s waking systems; conversely, the waking systems can turn off the sleep switch. “When you have a switch where either side can turn off the other, it’s what electrical engineers call a ‘flip-flop,’” Saper explains. “It likes to be in one state or the other. So we fall asleep, or wake up, quite quickly. Otherwise we’d be half asleep or half awake all the time, with only brief periods of being fully awake or asleep. But we’re not—we are either awake or asleep.” The adenosine cycle at least partly explains the homeostatic drive for sleep—the longer we are awake, the greater our fatigue, and pressure to sleep builds up progressively. But circadian rhythms also profoundly affect sleep and wakefulness. Circadian cycles (from circa, meaning “about,” and dies, a “day”) are internal periodic rhythms that control many things like body temperature, hormone levels, sleep and wakefulness, digestion, and excretion. “The circadian cycles go way back in evolutionary time,” Charles Czeisler says. “They are probably older than sleep.” Since the 1970s, Czeisler has established himself as one of the world’s leading authorities on circadian cycles and the chronobiology of sleep and wakefulness. He has done groundbreaking work in the sleep laboratory at Brigham and Women’s Hospital, where a special wing on one floor is shielded not only from sunlight, but from all external time cues. There, researchers can do exotic things like simulate the 708-hour lunar day or conditions on the International Space Station, where the sun rises and sets every 90 minutes. (Czeisler leads a sleep and chronobiology team that, under the auspices of NASA, researches human factors involved in space travel.) Exotic light environments like space challenge human biology, partly because people differ from other mammals, which take short catnaps and rat naps throughout the day and night. In contrast, we have one bout of consolidated (unbroken) sleep, and one of consolidated waking, per day (or, in siesta cultures, two of each). In addition, “There is a very narrow window [in the daily cycle] in which we are able to maintain consolidated sleep,” Czeisler says, “and the window gets narrower and narrower as we get older.” The origins of humans’ consolidated sleep take us to the beginnings of terrestrial life, since even prokaryotes—one-celled organisms like bacteria, lacking a nucleus—have built-in 24-hour rhythms. It is not surprising that these biological clocks are so universal, as they reflect the entrainment of all living things to the primeval 24-hour cycles of light and darkness created by the rotation of Earth. “The light and dark cycle is the most powerful synchronizer of the internal circadian clock that keeps us in sync with the 24-hour day,” Czeisler says. As late as 1978, when he published a paper demonstrating this effect, many still believed that “social interaction was the most important factor in synchronizing physiological cycles—that we had evolved beyond light,” he says. “But much of our subsequent research shows that our daily cycles are more like those of cockroaches than we want to believe. We are very sensitive to light.” Light strongly affects the suprachiasmatic nucleus (SCN), a biological clock in the anterior region of the hypothalamus that directs circadian cycles. All cells have internal clocks—even cells in a tissue culture run on 24-hour cycles. “They all oscillate like violins and cellos, but the SCN is the conductor that synchronizes them all together, ” Czeisler explains. While the homeostatic pressure to sleep starts growing the moment we awaken, the SCN calls a different tune. Late in the afternoon, its circadian signal for wakefulness kicks in. “The circadian system is set up in a beautiful way to override the homeostatic drive for sleep,” Czeisler says. The circadian pacemaker’s signal continues to increase into the night, offsetting the build-up of homeostatic pressure and allowing us to stay awake well into the evening and so achieve our human pattern of consolidated sleep and wakefulness. (There is often a dip in the late afternoon, when the homeostatic drive has been building for hours but the circadian signal hasn’t yet kicked in; Czeisler calls this “a great time for a nap.”) The evolutionary benefit of consolidated sleep and wakefulness is a subject of speculation; Czeisler says that long bouts of wakefulness may enable us to “take advantage of our greater intellectual capacity by focusing our energy and concentration. Frequent catnaps would interrupt that.” The circadian pacemaker’s push for wakefulness peaks between about 8 and 10 p.m., which makes it very difficult for someone on a typical schedule to fall asleep then. “The period from two to three hours before one’s regular bedtime, we call a ‘wake maintenance zone,’ ” Czeisler says. But about an hour before bedtime, the pineal gland steps up its secretion of the hormone melatonin, which quiets the output from the SCN and hence paves the way for sleep. Some years ago, melatonin supplements became popular as a natural sleeping pill, but as Czeisler’s research has proven, light is a more powerful influence on the biological clock than melatonin. Mangelsdorf professor of natural sciences J. Woodland Hastings has shown that even a split-second of light exposure can shift the circadian cycle of a single-celled organism by a full hour. Light interferes with sleep, at least partly because it inhibits melatonin secretion and thus resets the biological clock. For this reason, those seeking a sound sleep should probably keep their bedroom as dark as possible and by all means avoid midnight trips to brightly lit bathrooms or kitchens; blue light, with its shorter wavelength—and its resemblance to the sunlit sky—has the most powerful resetting effect. Light resets the pacemaker even in the case of some completely blind people, who generally lose circadian entrainment and suffer recurrent insomnia. “The eye has two functions, just as the ear does, with hearing and balance,” says Czeisler. “The eye has vision, and also circadian photoreception.” A subset of about 1,000 photosensitive retinal ganglion cells connects by a direct neural pathway to the SCN; these cells are sometimes active even in those who are blind to light. Exposure to bright light will decrease melatonin levels in some blind persons, and this subset, unlike other blind people, generally do not suffer from insomnia and are biologically entrained to the 24-hour day. Source: Harvard Magazinehttp://harvardmagazine.com/2005/07/deep-into-sleep.html

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