Scientists are learning more about our【C1】______ for sleep. Most people sleep【C2】______ ei

ght hours each night.【C3】______ people sleep more than【C4】______ and others sleep as【C5】______ as two or three hours each night.【C6】______ do not know exactly【C7】______ some people sleep more than others. Dr. Ernest Hartman has an idea about this. He believes the【C8】______ of sleep depends on how a person attacks problems. He said people who need only a few【C9】______ sleep usually are people who have much energy and make good use【C10】______ time to get their work【C11】______ quickly.【C12】______ he said many people who sleep longer than normal de creative work and seem to need【C13】______ dreaming time to find【C14】______ to emotional problems. Some scientists agree with this idea and others dispute.

To determine the【C15】______ of the lack of sleep, scientists have put subjects【C16】______ a set of psychological and performance tests【C17】______ them, for instance,【C18】______ 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【C19】______ , as are abilities to【C20】______ decisions and to concentrate."

【C1】

A．request

B．need

C．want

D．fancy