Scientists are learning more about our (67) for sleep. Most people sleep (68) eight hours each night. (69) people sleep more than (70) and others sleep as (71) as two or three hours each night. (72) do not know exactly (73) some people sleep more than others. Dr. Ernest Hartman has an idea about this. He believes the (74) of sleep depends on how a person attacks problems. He said people who need only a few (75) sleep usually are people who have much energy and make good use (76) time to get their work (77) quickly. (78) he said many people who sleep longer than normal de creative work and seem to need (79) dreaming time to find (80) to emotional problems. Some scientists agree with this idea and others dispute.
To determine the (81) of the lack of sleep, scientists have put subjects (82) a set of psychological and performance tests (83) them, for instance, (84) columns of numbers or recall a passage read to them only minutes earlier, "We’ve found that if you’re in sleep deficit (缺乏), performance suffers," says Dr. David. "Short-term memory is (85) , as are abilities to (86) decisions and to concentrate.\

A．And

B．While

C．Thus

D．But