It’s a catchy phrase: You snooze, you lose. But cutting out those 40 winks would be a bad idea. All mammals sleep, fish sleep, birds sleep and even fruit flies sleep. If an animal is deprived of sleep they die - faster than if they’re denied food.

Obviously, sleep rests the body, but in a different way than watching TV does. The body grows and heals during sleep. The body's metabolic system shifts into an anabolic phase of the metabolic cycle. Growth, healing and muscle building all take place when the body is in its anabolic phase - sleeping. The body's immune system functions much better when we sleep. One leading theory says that enzyme balances cause states of wakefulness and sleep. 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.

Sleep used to be studied by behaviorists. Their theories of what sleep is focused upon behavior. This prevented sleep from being understood. Recently sleep has begun to be understood at the metabolic level. At its most fundamental level, sleep seems to be a metabolic imperative. One guest below has suggested that sleep may well be the 'default' mode of life, and that being awake is little more than a period of heightened awareness of one's surrounding which is especially suited to doing things like finding food and reproducing. Indeed, the question "Why do we wake up" makes much more sense. It's an easily answerable question, too. For one thing, the body's metabolic system shifts into the catabolic phase. The body's cells are tearing themselves down in support of the body's need for energy to be mobile, to obtain food and to procreate. It's something that the body can only tolerate for a matter of several hours. Then the body must revert to it's 'default' mode of sleep, the anabolic phase when damage is repaired, growth can take place and the body's heightened immuned defenses intensify their battle against the foreign organisms and viruses that have invaded the animal.

Each night, about a quarter, is given to REM sleep, during which the brain is anything but idle. REM stands 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.

It's possible that sleep, generally, is the way that our mind transfers short term memory (RAM) into long term brain and muscle memory (Disk). This would help explain many phenomena, from the superior performance of students who sleep rather than "cram" prior to a test, to the documented importance of repetition across sleep cycles as a way to train the mind and body for athletic and musical performance.

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.