Sleep is far from wasted time. During a typical night the brain cycles several times through lighter and deeper stages, including periods of rapid eye movement when most vivid dreaming occurs. Deep sleep allows the body to repair tissue, release growth hormone, and strengthen the immune system. Meanwhile the brain sorts through the day's experiences, reinforcing important memories and discarding trivial ones. A cleaning process also runs more actively during sleep, flushing away waste products that build up between cells while we are awake.
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