A state which alternates with wakefulness, and in which awareness and responsiveness to the environment are reduced. It is not, however, uniform and can be divided into two main states, differentiated according to electrical recordings of brain activity (EEG), of the muscles (EMG), and of the eye movements (EOG).
This is subdivided into four stages, of which stage 1 is the lightest and stage 4 the deepest. The activity of the cerebral cortex (see BRAIN) is diminished and the body's functions are mainly regulated by brain-stem activity. The metabolic rate is reduced; in keeping with this the temperature falls, respiration is reduced, cardiac output, heart rate, and blood pressure fall, and activity of the sympathetic nervous system is reduced. NREM sleep normally occurs at the onset of sleep except in neonates. During adult life, the duration – particularly of stages 3 and 4 – of NREM sleep becomes less, and very little of this deep sleep occurs after the age of 60 years.
NREM sleep is thought to have several functions, such as energy conservation and growth. Growth hormone is produced in bursts during stages 3 and 4, and more cell division occurs during this type of sleep than during wakefulness. A controversial proposal has been that processing of information acquired during wakefulness occurs during NREM sleep.
This is characterised by the presence of rapid eye movements and a reduction in muscle tone. Cerebral cortical activity is prominent and its blood flow increased. This activity is, however, different from wakefulness and may cause irregular movements of the body as well as of the eyes. Most dreams occur in REM sleep: these may represent a process of reorganising mental associations after the period of wakefulness.
Physiological changes, such as a fall in temperature and blood pressure, take place just before sleep and continue during the early stages of NREM sleep. There is an intrinsic rhythm of sleep which in most subjects has a periodicity of around 25 hours. This can be modified by external factors to bring it into line with the 24-hour day. Two peaks of a tendency to sleep have been identified, and these usually occur between around 14.00–18.00 hours, and 02.00–06.00 hours. There are, however, differences according to age; for instance, infants sleep for most of the 24 hours; during adolescence there is also an increase in the duration of sleep. Sleep requirements fall later in life, but there are wide genetic differences in the amount of sleep that people require and also the time at which they fall asleep most readily.
The internal clock can be disturbed by a variety of external factors which include irregular sleeping habits due, for instance, to shift work or jet lag. Sleep is also more likely to occur after physical exertion, reading and social activity. The duration and intensity of exposure to light can also modify sleep profoundly. Light promotes wakefulness and is the main factor that adjusts the 25-hour internal rhythm to the 24-hour daily cycle. Neural connections from the retina of the EYE act on an area in the brain called the supra-chiasmatic nucleus, which stimulates the pineal gland which produces MELATONIN. This is thought to trigger the range of neurological and metabolic processes that characterise sleep.