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Sleep (slēp):

    A natural state of rest, occurring at regular intervals, in which the eyes usually close, the muscles relax, and

   responsiveness to external events decreases. Growth and repair of the tissues of the body are thought to occur duringsleep, and energy is conserved and stored. In humans and some other animals, scientists have identified one phase of sleep (called REM sleep) as the phase in which dreams occur.

    Did You Know? Shakespeare had it right. He said that sleep was the "balm of hurt minds" and that sleep "knits up the ravel'd sleeve of care." In other words, sleep helps overcome the stress of everyday life. So the third of your life you spend asleep is not a waste of time. All warm-blooded animals have the need to sleep. Studies have shown that animals that are not allowed to sleep for a long enough time can actually die. Babies, human and animal, sleep even more than adults do. Researchers think that babies may sleep so much because it helps the young body continue to develop quickly. Not only are babies' bodies growing, but their brains are, too – and sleep is very important for the brain. During sleep, the brain sorts through experiences and stores important new information for later use. This processing of experiences, in fact, is thought to be a major source of dreams.

(Source: The American Heritage® Student Science Dictionary, Second Edition. Copyright © 2014 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.)