Sleep

Sleep, state of relative unconsciousness and inactivity. The need for sleep recurs periodically in all animals. If deprived of sleep, humans initially experience hallucinations and acute anxiety and become highly suggestible. Eventually coma and sometimes death result. During sleep, the body is relaxed and most bodily activity is reduced. Cortical, or higher, brain activity, is measured by the electroencephalograph; blood pressure, body temperature, and rate of heartbeat and breathing are decreased. However, certain activities, such as gastric and alimentary activity, are increased. Sleep tends to occur in daily cycles that exhibit up to 5 or 6 periods of deepness—alternating with periods of paradoxical, or rapid-eye-movement (REM), sleep, characterized by restlessness and jerky movements of the eyes. Paradoxical sleep occurs only when we are dreaming and occupies about 20% of total sleeping time. Sleepwalking (somnambulism) occurs only during orthodox sleep, when we are not dreaming. Sleeptalking occurs mostly in orthodox sleep. Many theories have been proposed to explain sleep. Separate sleeping and waking centers in the hypothalamus cooperate with other parts of the brain in controlling sleep. Sleep as a whole—and particularly paradoxical sleep, when dreaming occurs—is essential to health and life. Consequently, the key to why animals sleep may reside in a need to dream.