Sleep is the state of natural rest observed throughout the animal kingdom — all mammals and birds, and in many reptiles, amphibians, and fish. 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 of arousal from sleep is a protective mechanism that is also necessary for health and survival.
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Sleep is generally characterized by a reduction in voluntary body movement, decreased to little reaction to external stimuli, loss of consciousness, reduction in audio receptivity, an increased rate of anabolism (the synthesis of cell structures), and a decreased rate of catabolism (the breakdown of cell structures).
In mammals, the measurement of eye movement during sleep is used to divide sleep into two categories or phases: rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. Each category has a distinct set of associated physiological, neurological and psychological features.
Sleep occurs in cycles of REM and NREM phases. In humans, this cycle is approximately 90 to 120 minutes. Each phase may have a distinct physiological function. Drugs such as alcohol and sleeping pills can suppress certain stages of sleep (see Sleep deprivation). This can result in a sleep that exhibits loss of consciousness but does not fulfill its physiological functions.
In the REM phase, 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. Also during REM sleep our muscles are paralyzed so that we don’t act out our dreams.
In the NREM sleep phase, the body is active and the brain is 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 silence, or “drowsy sleep”. Associated with the onset of sleep during N1 may be sudden twitches and hypnic jerks. These are normal, as is an increased instance of flatulence. Other people may also experience hypnagogic hallucinations during this stage, which can be more troublesome to the subject. During N1 the subject loses some muscle tone, and conscious awareness of the external environment.
Stage N2, is characterized by “sleep spindles” (12 to 16 Hz) and “K-complexes.” During this stage the electromyography (EMG) lowers, and conscious awareness of the external environment disappears. This occupies 45 to 55% of total sleep.
In Stage N3, the delta waves, also called delta rhythms (0.5 to 4 Hz) make up less than 50% of the total wave-patterns. This is considered part of the slow-wave sleep (SWS) and functions primarily as a transition into stage N4. Overall it occupies 3 to 8% of total sleep time. This is the stage in which night terrors, bed wetting, sleepwalking, and sleep-talking occur.
In Stage N4, delta-waves make up more than 50% of the wave-patterns. Stages N3 and N4 are the deepest forms of sleep; N4 is effectively a deeper version of N3, in which the deep-sleep characteristic, such as delta-waves, are more pronounced.
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