Humans spend about a third of the time asleep but why we sleep is not yet fully understood. Sleep is a state of inactivity accompanied by loss of awareness and a markedly reduced responsiveness to environmental stimuli. When a recording is made of the electroencephalogram (EEG) and other physiological variables such as muscle activity and eye movements during sleep (a technique called polysomnography), a pattern of sleep emerges, consisting of five different stages. This pattern varies from person to person, but usually consists of four or five cycles of quiet sleep alternating with paradoxical, or active, rapid eye movement (REM) sleep, with longer periods of paradoxical sleep in the latter half of the night. A representation of these stages and cycles over time is known as a hypnogram, and one derived from a normal subject appears in Figure 19.6, with paradoxical sleep depicted as the shaded areas.
Quiet sleep is further divided into four stages, each with a characteristic EEG appearance, during which there is progressive relaxation of the muscles and slower, more regular breathing as the deeper stages are reached. Most sleep in these deeper stages occurs in the first half of the night.
During paradoxical sleep, the EEG appearance is similar to that of waking or drowsiness. There is irregular breathing, complete loss of tone of the skeletal muscles, and frequent phasic movements particularly of the eyes, consisting of conjugate movements which are mostly lateral but can also be vertical (hence the term rapid eye movement sleep); most dreaming takes place in this stage.
The length of total sleep in a day varies between 3 and 10 hours in normal subjects with an average in the 20-45 year age group of 7-8 h. Sleep time is decreased in older subjects, to about 6 h in the over 70 year age group, with increased daytime napping reducing the actual night time sleep even more. The amount of time spent in each of the five stages varies between subjects and particularly with age, with much less slow wave sleep in older people. The number of awakenings after the onset of sleep also increases with advancing age. A normal subject has several short awakenings during the night, most of which are not perceived as awakenings unless they last more than about 2 minutes. Probably there will not be clear consciousness but subject may have occasional brief thoughts of how comfortable
Awake Movement REM Stage 1 Stage 2 Stage 3 Stage 4
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