Judging from recent surveys,most experts in sleep behavior agree that there is virtually an epidemic (流行病) of sleepiness in the nation. “I can'tthink of a single study that hasn't found Americans getting less sleep thanthey ought to,” says Dr. David. Even people who think they are sleeping enoughwould probably be better off with more rest.

　　The beginning of our sleep-deficit (睡眠不足) crisis can betraced to the invention of the light bulb a century ago. From diary entries andother personal accounts from the 18th and 19th centuries, sleep scientists havereached the conclusion that the average person used to sleep about 9.5 hours anight. “The best sleep habits once were forced on us, when we had nothing to doin the evening down on the farm, and it was dark.” By the 1950s and 1960s, thatsleep schedule had been reduced dramatically, to between 7.5 and eight hours,and most people had to wake to an alarm clock. “People cheat on their sleep,and they don't even realize they're doing it,” says Dr. David. "They thinkthey're okay because they can get by on 6.5hours, when they really need 7.5, eight or even more to feel ideallyvigorous."

　　Perhaps the most merciless robber of sleep,researchers say, is the complexity of the day. Whenever pressures from work,family, friends and community mount, many people consider sleep the leastexpensive item on his programme. 'In our society, you're considered dynamic ifyou say you only need 5.5 hours' sleep. If you've got to get 8.5 hours, peoplethink you lack drive and ambition."

　　To determine the consequences of sleep deficit,researchers have put subjects through a set of psychological and performancetests requiring them, for instance, to add columns of numbers or recall apassage read to them only minutes earlier. "We've found that if you're insleep deficit, performance suffers," says Dr. David. “Short-term memory isweakened, as are abilities to make decisions and to concentrate.”

　　1. People in the 18th and 19th centuries used to sleep about 9.5 hoursa night because they

　　had _______ .

　　A) no drive and ambition B) noelectric lighting

　　C) the best sleep habits D)nothing to do in the evening

　　2. According to Dr. David, Americans _______ .

　　A) areideally vigorous even under the pressure of life

　　B) ofienneglect the consequences of sleep deficit

　　C) do notknow how to relax themselves properly

　　D) can getby on 6.5 hours of sleep

　　3. Many Americans believe that _______ .

　　A) sleep isthe first thing that can be sacrificed when one is busy

　　B) they needmore sleep to cope with the complexities of everyday life

　　C) to sleepis something one can do at any time of the day

　　D) enoughsleep promotes people's drive and ambition

　　4. The word “subjects” (Line 1, Para. 4) refers to______ .

　　A) the performancetests used in the study of sleep deficit

　　B) specialbranches of knowledge that are being studied

　　C) peoplewhose behavior or reactions are being studied

　　D) thepsychological consequences of sleep deficit

　　5. It can be concluded fromthe passage that one should sleep as many hours as is necessary to ______ .

　　A) improveone's memory dramatically

　　B) beconsidered dynamic by other people

　　C) maintainone's daily schedule

　　D) feelenergetic and perform adequately

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