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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 main idea of the passage is to______.
选项 A、illustrate what enough sleep is through tests
B、explain that cognitive performance remains intact with less sleep
C、clarify that countermeasures can help to release sleepiness
D、demonstrate ways to make up for sleep deprivation
答案A
解析 主旨题。这篇文章主要介绍丁格斯所进行的实验。为了知道充足的睡眠到底是多长时间，科学家们进行了多种有关睡眠的实验，将受试者们分成了不同的小组，来测试在不同睡眠情况下他们注意力的维持程度。通过实验得知，八小时睡眠是比较充足的睡眠，而不充足的睡眠会导致人注意力下降，损害认知能力。由此可知，[A]“通过测试说明何为足够睡眠”是正确答案。[B]“解释即使睡眠不足，认知能力依然不会损伤”与文章大意不一致，故排除；[C]“说明有效措施可以缓解睡意”只是在第七段有所涉及，不能作为文章大意，故排除；[D]“说明弥补被剥夺睡眠的方法”，在文章最后一段指出用周末弥补睡眠债是否有效，到现在并没有确切答案，故排除。
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