As the next day starts to dawn the previously overwhelming desire to sleep grows weaker and weaker. By mid-morning you might even feel as if nothing out of the ordinary had happened.

A reasonable question to ask, then, is this: if we can stay up all night and not have to pay for it, why not stay up the next night as well -- and the next? Why can't we put off sleeping indefinitely?

One answer lies in those nightly psychic events called dreams. Research has shown that the brain actually needs to dream, as can be seen in a simple phenomenon known as "REM Rebound." REM refers to the stage of sleep in which most dreams occur. When volunteer subjects stay awake one night -- and thus miss out on REM -- and then sleep the second night, they spend twice as much time dreaming as they normally would have. People deprived of two nights' sleep dream even more on the third night, and so on. Selectively waking up subjects only when they go into REM sleep results in the same phenomenon -- apparently the brain doesn't just need to sleep, it needs to dream.

Now it isn't true, as is sometimes said, that you will go insane if not allowed to dream. What is true, however, is that dreams are by no means an unimportant part of mental life; indeed, Rem Rebound suggests that a good night's dream is just as important as a good night's rest.  

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