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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.
It can be learned from Paragraph Five that________.
选项 A、8-hour subjects fared the best among three groups
B、the 6-hour subjects began to decline in the middle of the study
C、half of the 6-hour subjects fell asleep at the computer
D、4-hour subjects were more impaired than 6-hour subjects
答案A
解析 推断题。根据题干提示定位到第五段。该段主要讲述了丁格斯进行的一项最长睡眠限制期的研究结果。该段第一、二句指出，在为期14天的实验中，达到八小时睡眠的受试者几乎没有出现任何注意力分散或认知能力下降的现象，四小时组的受试者情况则很糟，还有六小时组参与者的表现也在走下坡路。[A]“八小时睡眠的受试者在三个组中进展最好”与之相符，是正确答案。该段第二句指出，四小时与六小时组的受试者的表现是日复一日逐步下降的，并不是到了实验中期才开始下降的，[B]“六小时组的受试者在实验进行到一半时表现开始下降”与文意不符，故排除；该段第四句指出，到了第六天时，六小时组中25％的人在电脑前进入了梦乡，[C]“六小时组的一半受试者都在电脑前睡着了”与文意不一致，故排除；[D]“四小时组受试者比六小时组受试者受损程度大”，文中并没有涉及二者之间的比较，故排除。
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