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  3. We all know that we don’t get enough sleep. But how much sleep do we really need? Until about 15 years ago, one common theory wa

We all know that we don’t get enough sleep. But how much sleep do we really need? Until about 15 years ago, one common theory wa

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问题     We all know that we don’t get enough sleep. But how much sleep do we really need? Until about 15 years ago, one common theory was that if you slept at least four or five hours a night, your cognitive performance remained intact; your body simply adapted to less sleep. But that idea was based on studies in which researchers sent sleepy subjects home during the day—where they may have sneaked in naps and downed coffee.
    Enter David Dinges, the head of the Sleep and Chronobiology Laboratory at the Hospital at University of Pennsylvania, who has the distinction of depriving more people of sleep than perhaps anyone in the world. In what was the longest sleep-restriction study of its kind, Dinges and his lead author, Hans Van Dongen, assigned dozens of subjects to three different groups for their 2003 study: some slept four hours, others six hours and others, for the lucky control group, eight hours— for two weeks in the lab.     Every two hours during the day, the researchers tested the subjects’ ability to sustain attention with what’s known as the psychomotor vigilance task, or P. V. T. , considered a gold standard of sleepiness measures. During the P. V. T. , the men and women sat in front of computer screens for 10-minute periods, pressing the space bar as soon as they saw a flash of numbers at random intervals. Even a half-second response delay suggests a lapse into sleepiness, known as a microsleep.
    The P. V. T. is tedious but simple if you’ve been sleeping well. It measures the sustained attention that is vital for pilots, truck drivers, astronauts. Attention is also key for focusing during long meetings; for reading a paragraph just once, instead of five times; for driving a car. It takes the equivalent of only a two-second lapse for a driver to veer into oncoming traffic.
    Not surprisingly, those who had eight hours of sleep hardly had any attention lapses and no cognitive declines over the 14 days of the study. What was interesting was that those in the four-and six-hour groups had P. V. T. results that declined steadily with almost each passing day. Though the four-hour subjects performed far worse, the six-hour group also consistently fell off-task. By the sixth day, 25 percent of the six-hour group was falling asleep at the computer. And at the end of the study, they were lapsing fives times as much as they did the first day. The six-hour subjects fared no better—steadily declining over the two weeks—on a test of working memory in which they had to remember numbers and symbols and substitute one for the other. The same was true for an addition-subtraction task that measures speed and accuracy. All told, by the end of two weeks, the six-hour sleepers were as impaired as those who, in another Dinges study, had been sleep-deprived for 24 hours straight—the cognitive equivalent of being legally drunk.
    So, for most of us, eight hours of sleep is excellent and six hours is no good, but what about if we split the difference? What is the threshold below which cognitive function begins to flag? While Dinges’s study was under way, his colleague Gregory Belenky, then director of the division of neuroscience at the Walter Reed Army Institute of Research in Silver Spring, Md. , was running a similar study. He purposely restricted his subjects to odd numbers of sleep hours—three, five, seven and nine hours—so that together the studies would offer a fuller picture of sleep-restriction. Belenky’s nine-hour subjects performed much like Dinges’s eight-hour ones. But in the seven-hour group, their response time on the P. V. T. slowed and continued to do so for three days, before stabilizing at lower levels than when they started. Americans average 6. 9 hours on weeknights, according to the National Sleep Foundation. Which means that, whether we like it or not, we are not thinking as clearly as we could be.
    Of course our lives are more stimulating than a sleep lab: we have coffee, bright lights, the social buzz of the office,all of which work as "countermeasures" to sleepiness. They can do the job for only so long, however. As Belenky, who now heads up the Sleep and Performance Research Center at Washington State University, Spokane, where Van Dongen is also a professor, told me about cognitive deficits:"You don’t see it the first day. But you do in five to seven days. "
    And it’s not clear that we can rely on weekends to make up for sleep deprivation. Dinges is now running a long-term sleep restriction and recovery study to see how many nights we need to erase our sleep debt. But past studies suggest that, at least in many cases, one night alone won’t do it.
The old common theory was that if you slept at least four or five hours a night,______.
选项 A、your performance would be poor because of cognitive deficits
B、you would fall asleep in front of the computer
C、you would become energetic because your life was stimulating
D、you would work normally because your body could adapt to it
答案D
解析 细节题。根据题干关键词“common theory”定位到第一段。该段第三句提到，大约在15年前，有一种达成共识的理论，那就是如果你每晚至少能保持四至五个小时的睡眠，你的认知能力就不会受到损伤，你的身体完全能够适应较少的睡眠．[D]“你能够正常工作因为你的身体能够适应它”与之相符，是正确答案。[A]“由于认知障碍，工作表现将很差”，在第三句中已经说明了认知能力不会受到损伤，不会产生认知障碍，故排除；[B]“将会在电脑前睡着”在文中并没有提及，故排除；[C]“由于生活很刺激，你将会很有活力”，在共识中没有涉及此内容，故排除。
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