While many of us think of sleep as a largely forgotten stretch of time when nothing happens, sleep is, neurologically speaking at least, a busy time indeed. Although sleep's importance is inarguable, scientists do not know exactly why it is so important to our survival. What happens when we sleep?

Sleep Stages
Scientists recognize sleep stages by tracking the changes in brain waves. The five sleep stages are repeated as many as five times during the night. As the night progresses, each cycle lengthens, and REM sleep, during which most dreaming takes place, extends.

• Sleep Stage 1
  In this brief stage, which may last only a few minutes, the body drifts to sleep. Brain waves are mostly high amplitude, slow waves and occasional alpha waves (like those found when awake).
  Percent of total sleep time for young adults: 5 percent.

• Sleep Stage 2
  Heartbeat and breathing slow and the sleep is deeper than in Stage 1. Slow-wave sleep continues with peaks of brain waves (known as sleep spindles) occurring.
  Percent of total sleep time for young adults: 44 to 55 percent.

• Sleep Stages 3 and 4
  These are the stages of deepest sleep, when brain waves are slowest. During these stages breathing and heartbeat slow further and muscles relax. Dreams are more common than in the earlier stages and sleepwalking and talking may occur during Stages 3 and 4.
  Percent of total sleep time for young adults: 15 to 23 percent.

• REM
  Rapid eye movement (REM) sleep stages lengthen through the night. The first REM cycle may be only 10 minutes while the last could last as long as an hour. During this cycle the heartbeat increases, breathing becomes shallow, eyes move rapidly, muscles are relaxed, and dreams are most vivid. Brain waves resemble those during waking.
  Percent of total sleep time for young adults: 20 to 25 percent.

Why Do We Sleep?
Sleep is essential to life. Laboratory animals deprived of sleep die. And humans don't seem to be immune to the life-threatening effects of sleeplessness. A survey conducted by the American Cancer Society concluded that people who sleep 6 hours or less per night, or who sleep 9 hours or more, had a death rate 30 percent higher than those who regularly slept 7 to 8 hours. Even those who slept 6 hours or less who otherwise had no health problems had death rates 1.8 times higher than those who slept "normal" hours.

But what is the purpose of sleep? Despite the obvious nature of that question, scientists do not really agree on why we sleep. There are several theories:

• Adaptive Theory
  This theory holds that sleep improves an animal's likelihood of survival. Those with sleeping habits appropriate to their environment are most likely to survive. Nocturnal species have very different sleep habits than diurnal hunters, for example, making them more likely to flourish.
• Energy Conservation Theory
  Fast-moving animals with high metabolisms sleep more than those that burn calories more slowly, thereby conserving their energy for sprints.
• Restorative Theory
  According to this theory, the body restores itself during sleep. Researchers know that neurotoxins are neutralized during sleep, and have reported that cells divide, tissue synthesizes and growth hormones are released during slow-wave (or non-REM) sleep. Athletes, for example, spend more time in slow-wave sleep (Stages 3 and 4) than others, and children and young people spend a larger portion of their sleep in slow-wave sleep than older people.
• Programming-Reprogramming Theory
  This theory holds that unimportant information is "erased" and important information is locked into more permanent memory. Infants, who are acquiring information at a rate faster than at any other point during life, sleep most. All sleep may not be equal for reinforcing learning, however. Recent research indicates that REM sleep may be the key. Babies and children experience a larger portion of REM sleep than adults, and adults who are in school or undergoing intense intellectual training increase their amount of REM sleep. When people are deprived of REM sleep they are less adept at creative problem solving.

Circadian Rhythms
Our biological "clock" largely corresponds to the cycle of the day, and in fact the term "circadian" means "about a day." The cycle of wakefulness and sleep is tied closely to core body temperature: the higher the temperature, the more alert we are; conversely, when it reaches its low point, sleepiness may be irresistible. The body's rhythms seem based on two sleep periods each day: a long one through the night and a second short period in the afternoon, when many people nap or at least feel less alert than at other points during the day.

Researchers have found that when people are removed from any outside reminders of time (no clocks, no outside light, etc.) their "clock" seems to be set approximately for a 25-hour day. When one's personal biological clock gets out of sync with society's clock, sleep problems can ensue.