In humans, other mammals, and many other animals that have been studied — such as fish, birds, ants, and fruit-flies — regular sleep is necessary for survival. The capability for arousal from sleep is a protective mechanism and also necessary for health and survival.

In mammals, the measurement of eye movement during sleep is used to divide sleep into two broad types: rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. Each type has a distinct set of associated physiological, neurological and psychological features.

Sleep proceeds in cycles of REM and NREM phases. In humans, this cycle is approximately 90 to 110 minutes. Each phase may have a distinct physiological function. Drugs such as alcohol and sleeping pills can suppress certain stages of sleep. This can result in a sleep that exhibits loss of consciousness but does not fulfill its physiological functions.

In REM, the brain is active and the body inactive, and this is when most dreaming occurs. REM sleep is characterized by an electroencephalography (EEG) that has low voltage and mixed frequency, similar in appearance to the wakeful EEG. During REM sleep there is loss of skeletal muscle tone, and an active sympathetic nervous system.

In NREM sleep, the body is active and the brain inactive, and there is relatively little dreaming. Non-REM encompasses four stages; stages 1 and 2 are considered 'light sleep', and 3 and 4 'deep sleep'. They are differentiated solely using EEG, unlike REM sleep which is characterized by rapid eye movements and relative absence of muscle tone. There are often limb movements, and parasomnia sleep walking occurs in non-REM sleep. A cyclical alternating pattern may sometimes be observed during a stage.