A、5.5 hours. B、Between 7.5 and 8 hours. C、6.5 hours. D、8. 5 hours. B

Judging from recent surveys, most experts in sleep behavior agree that there is virtually an epidemic of sleepiness in the United States. The beginning of this sleep-deficit crisis can be traced to the invention of light bulb a century ago. In the 18th or 19th century, the average person used to sleep about 9. 5 hours a night. The best sleep habits were forced on people of that time, because it was dark and people had nothing to do in the evening on the farm.
      By the 1950s and 1960s, the sleep schedule had been reduced dramatically to between 7.5 and 8 hours on average. Most people had to wake to an alarm clock. People cheat on their sleep, but they don’t realize they are doing it. They think they are OK because they can get by on 6.5 hours, when they really need 8 hours or even more to feel ideally vigorous.
     Perhaps the most merciless robber of sleep, researchers say, is the complexity of the day. Whenever pressure from work, study and family, etc. mounts, many people consider sleep the least expensive item on his program. In the United States, one is considered dynamic if he says he only needs 5 hours’ sleep, whereas if he has to get 8.5 hours, people think he lacks drive and ambition.
     The consequences of sleep deficit are obvious. When researchers put subjects through a set of psychological and performance tests, they found that people in sleep deficit showed weakened short-term memory as well as poor abilities to make decisions and to concentrate.