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Why Do We Sleep?

It's a catchy phrase: You snooze, you lose. But cutting out those 40 winks would be a bad idea. All mammals sleep, and if they're deprived of shut-eye they die--faster than if they're denied food. But no one really knows why.
Obviously, sleep rests the body. But watching TV does that, too. The answer must lie in the noggin. One leading theory says that while we're awake, a substance builds up in the brain (or gets depleted) and sleep removes (or replenishes) it. That makes sense. For part of the night, the brain idles in an energy-conserving state called slow-wave sleep. Freed from the duties of consciousness, it can focus on cleanup.
The problem with this idea is that another portion of each night, about a quarter, is given to REM sleep, during which the brain is anything but idle. REM stand for rapid eye movement, and it corresponds with vivid dreams, suggesting that it plays a role in consolidating memories. But there's probably more to it: Though antidepressants suppress REM sleep, patients taking them suffer no memory impairment.
In any case, it's clear that pillow time serves a critical purpose. Bad things--like some 100,000 traffic accidents a year, not to mention uncounted instances of calling your spouse by your ex's name happen when we don't get enough z's. At some point, someone's going to have to dream up a reason.