单项选择题

Sleep is something we generally connect with living things. Of course, it is true that a tot of animals sleep, but scientists are not certain that primitive (原始的) forms of animal life like worms ever really sleep. On the other hand, animals such as bears sleep for 4 or 5 months every year.
The amount of sleep that a human being needs depends on age, tile individual and possibly race. For example, doctors think pre-school children need between 10 and 12 hours a night; school children between 9 and 11 hours; and adults between 7 and 9 hours. There are cases of old people who only sleep between 2 and 3 hours a day and continue to be active and healthy. The sleep requirements Of different races also appear to be different. Japanese people, for example, sleep fewer hours than Europeans.
It is not known whether mental activity (except dreaming) occurs when a person is asleep. However, it is certainly true that some people can wake up at a particular time. There are also stories about scientists who solve difficult problems during sleep, because their subconscious (潜意识的) minds continue working on the problem.
Some types of nightmares are quite common: the dreamer is falling from a tree; or an animal is running after him. Some people say these two occur because man’s ancestors (祖先) lived in trees and were in constant danger from wild animals.

We can infer from the text that().

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