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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.
Which of the following statements is NOT true about P. V. T. ?
选项 A、It is a standard of sleepiness measures.
B、It is an easy but thought-provoking test.
C、It can tell a lapse into sleepiness.
D、It can measure the sustained attention.
答案B
解析 推断题。根据题干关键词P．V．T．定位到第三、四段。第四段第一句提到，用P．V．T．法检测睡眠虽单调乏味但却简单有效，故[B]与原文意思不符，是正确答案。[A]“它是衡量睡眠充足程度的标准”与第三段第一句意思相符，故排除；[C]“它能辨明是否进入睡眠状态”与第三段第三句意思一致，故排除；[D]“它能够衡量注意力的持续”与第四段第二句意思一致，故排除。
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