Sleep

At one time, it was believed that the mind simply turned off during sleep, or that the soul left the body during sleep. Aristotle thought that the digestion of food created vapors which naturally rose upward, causing the brain to become drowsy. Dreams—the only part of sleep the sleeper actually experiences—were often interpreted as prophetic revelations. Today, dream interpretation is used in some psychoanalytic and self-awareness activities for personal insight and revelation.

Despite the fact that most people spend more time sleeping than in any other single activity, scientists still lack much knowledge about why we need sleep or what triggers it. Serious scientific studies only began about 50 years ago, and several different theories have been developed, none of which have been proven. It is known, however, that the higher the organism on the evolutionary chain (humans being the highest) the more important sleep becomes.

According to the restorative theory of sleep, body tissues heal and regenerate during non-REM sleep and brain tissue heals during REM sleep. This theory seems generally accepted for brain tissue restoration, particularly in the cerebral cortex, which cannot rest during the waking state. However, some researchers question its validity regarding body tissue restoration, believing that sleep simply acts as an immobilizer, forcing the body to rest, with rest and nourishment being the actual restorative factors. The release during sleep of growth hormones, testosterone, and other anabolic (constructive) hormones leads some experts to support the restorative theory, while others believe this release is coincidental to, and not caused by, sleep.

The energy conservation theory of sleep notes that animals which burn energy quickly and produce their own body heat, such as humans do, sleep more than those with slow metabolisms (energy consumption) or that do not produce body heat (snakes, for instance). This theory is based upon the observation that metabolic rates decrease during slow-wave sleep—the last two stages of the four-stage, NREM sleep cycle and which some researchers believe is the most important stage.

According to the adaptive theory of sleep, sleep encourages adaption to the environment for increased chances of survival. Animals such as cats that spend little time searching for food and have few natural enemies may sleep 15 hours a day for long periods. Grazing animals like buffaloes and horses which spend many hours foraging and which are at risk from natural predators sleep only two to four hours a day in short spurts. Proponents of the adaptive theory believe early humans slept in caves to protect themselves from night-stalking animals.

Because instinct plays an important role in the survival of any species, including humans, the instinct theory presumes sleep, like mating or hunger, is a survival instinct.

Studies show that new information is best retained when introduced just before sleep begins and retained less well after waking or if REM sleep is interrupted. These observations lead to the memory consolidation theory of sleep. REM sleep seems to play an important role in storing information.