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."

【26】

A．for

B．at

C．in

D．since