单项选择题

Judging from recent surveys, most experts in sleep behavior agree that there is virtually an epidemic of sleepiness in the nation. "I can’t think of a single study that hasn’t found Americans getting less sleep than they ought to." says Dr. David.
The beginning of our sleep deficit crisis can be  (1)  to the invention of the light bulb a century ago. From diary entries and other personal  (2)  from the 18th and 19th centuries, sleep scientists have reached the conclusion that the average person used to sleep about 9.5 hours a night. By the 1950s and 1960s, that sleep schedule had been reduced  (3) , to between 7.5 and 8 hours, and most people had to wake to an alarm clock. "People cheat on their sleep, and they don’t even  (4)  they’re doing it," says Dr. David. "They think they’re okay because they can get by on 6.5 hours, when they really need 7.5, 8 or even more to feel ideally  (5) ."
Perhaps the most merciless robber of sleep, researchers say, is the  (6)  of the day. Whenever pressures from work, family, friends and community mount, many people consider sleep the least  (7)  item on the agenda. "In our society, you’re considered dynamic if you say you only need 5.5 hours’ sleep, if you say you’ve got to get 8.5 hours, people think you lack  (8)  and ambition."
To assess the  (9)  of sleep deficit, researchers have put subjects through a set of psychological and performance tests requiring them. "We’ve found that if you’re sleep-deprived, performance  (10) ," says Dr. David. "Short-term memory is impaired, as are abilities to make decisions and to concentrate."

（1）处填（）。

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