Sleep is a BEHAVIOURAL STATE in mammals that is defined by recumbent postures, elevated sensory threshold and distinct polygraphic (see POLYGRAPH) signs. During sleep, there is little motor activity, the eyes are usually closed, and responses to external stimuli are greatly diminished compared to during wakefulness. There is also a loss of CONSCIOUSNESS and occasional DREAMING in humans. Unlike COMA, however, these conditions are reversible, and sleeping individuals or animals can be awakened by sufficiently strong stimuli. Sleep is not a homogeneous state but consists of two basic states: non-rapid eye movement sleep (NON-REM SLEEP, or SLOW-WAVE SLEEP) and REM SLEEP (rapid eye movement sleep). Non-REM sleep is classified further into four stages. These states and stages are best characterized by polygraphic recordings including the ELECTROENCEPHALOGRAM (EEG), ELECTROMYOGRAM (EMG) and ELECTRO-OCULOGRAM (EOG). Nocturnal sleep is organized in cycles, with a period of about 90 minutes in humans. Each cycle starts with wakefulness, which can be very brief or absent between cycles, and evolves through the stages of non-REM sleep and then REM sleep. REM sleep occurs only directly after stage 4 of non-REM sleep, and it will not occur if awakening follows non-REM sleep. Otherwise, reverse transitions between adjacent stages are not uncommon. In humans, the proportion of REM sleep in each cycle increases towards dawn. Dreaming occurs mostly during REM sleep, but some occurs during non-REM sleep.
At the same time, ascending pathways from the reticular formation activate the THALAMUS and forebrain structures to induce AROUSAL of the CEREBRAL CORTEX. The mechanisms for non-REM sleep are less well understood than for REM sleep. However, the PREOPTIC AREA in the HYPOTHALAMUS and the dorsal reticular formation of the MEDULLA including the NUCLEUSOF THE SOLITARY TRACT are thought to have a role in non-REM sleep. Humoural factors such as HORMONES, CYTOKINES and ADENOSINE have also been implicated in promotion of sleep. Sleep and wakefulness also show daily rhythms, and are controlled by the circadian clock (see BIOLOGICAL CLOCK; CIRCADIAN RHYTHM) located in the SUPRACHIASMATIC NUCLEUS of the hypothalamus. The circadian pacemaker does not appear to affect sleep and arousal equally, but rather, promotes and maintains wakefulness at particular times in the circadian cycle.
The function of sleep remains speculative. The function of non-REM sleep is thought to be the restoration of bodily functions. In support of this notion, non-REM sleep represents the largest portion of sleep time, and is associated with decreases in autonomic functioning such as heart rate, blood pressure and rate of respiration, as well as increased release of GROWTH HORMONE, PROLACTIN and Other hormones. In contrast, much less is known about the function of REM sleep. Unlike non-REM sleep, REM sleep is associated with elevated AUTONOMIC activity, although temperature regulation is lost. Penile erection occurs in males. Subjects awakened during REM sleep often report dreaming. These observations suggest that the brain is active during REM sleep. One hypothesis is that REM sleep helps to maintain the excitability of the cerebral cortex. This might have a role in the CONSOLIDATION of MEMORY in adults, and in stimulating the developing brain, as new-born babies sleep about 16 hours a day about half of which is spent in REM sleep. This cortical HOMEOSTASIS hypothesis is consistent with the view that REM sleep represents evolutionarily a more recent phenomenon than non-REM sleep because it is seen only in mammalian species.
KAZUE SEMBA
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