Every attempt to reveal the secrets of sleep, and be precise about its function, raises many problems. The sleeper himself cannot tell what is going on, and even when he wakes up, he has only a very confused idea of how good or bad a night he has had. The research is expensive and often unpopular, as it inevitably involves working at night. Only in the last few years have experts come up with theories about the function of sleep and the laws which may govern it.

(88 words)

　　What is the passage mainly about?
　　 A) The function of sleep.
　　 B) The secrets of sleep.　　　　
　　 C) The problems in the research on sleep.　　　　
　　 D) The success of the research on sleep.
　　　　

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