单项选择题

Scientists are learning more about our need for sleep. Most people sleep (26) eight hours each night. (27) people sleep more than (28) and others sleep as (29) as two or three hours each night. Scientists do not know exactly (30) some people sleep more than others. Dr. Ernest Hartman has a (an) (31) about this. He believes that the (32) of sleep depends on how a person (33) problems. He said people who need only a few (34) sleep usually are people who have much energy and make good use of (35) to get their work to be done quickly. (36) he said many people who sleep longer than normal do creative work and seem to need (37) dreaming time to find solution (38) emotional problems. Some scientists agree (39) this idea and others dispute.
To determine the (40) of the lack of sleep, scientists have put (41) through a set of psychological and performance tests (42) them, for instance, to add columns of numbers or recite a passage read to them only minutes earlier, "We’ve found that if you’re in sleep (43) , performance suffers," says Dr. David. "Short-term memory is (44) , (45) are abilities to make decisions and to concentrate.\