A periodic state of relative unconsciousness and inactivity from which a
person or animal can be roused. If deprived of sleep humans initially experience hallucinations, acute anxiety, and
become highly suggestible and eventually coma and sometimes death result.
During sleep, the body is relaxed and most bodily activity is reduced. Cortical, or higher brain activity, as measured by an electroencephalograph; blood pressure; body temperature; and rate of heart rate and breathing are decreased. However, certain activities, such as gastric and alimentary activity, are increased.
Sleep tends to occur in daily cycles which exhibit up to 5 or 6 periods of orthodox sleep – characterized by its deepness – alternating with periods of paradoxical, or rapid eye movement (REM) sleep, characterized by its restlessness and jerky movements of the eyes. It is during this REM sleep, which occupies about one fifth of total sleeping time, that dreaming occurs. Sleepwalking (somnambulism) occurs only during orthodox sleep and nit when we are dreaming.
Separate sleeping and waking centers in the hypothalamus cooperate with other parts of the brain in controlling sleep. A rested brain and concentration are probably the most effective basis for concentration. Difficulty in sleeping is called insomnia.