A state of total or partial unawareness ranging from slight wakefulness to light tranquility, to nearly total detachment from the external world.
Sleep is the time of rest and rejuvenation for the body. The muscles and the nerves relax. The body recharges its energy for the hours of work to come the next day. People may need a siesta in the afternoon too, depending upon the climate, culture, age and health status.
The stages of sleep
These stages move in cycles of about 90 minutes duration each. The person passes through regular transitions between these stages. The sleep becomes less and less deep as the night passes. These stages can be studied through an EEG.
Stage 1 sleep
The initial stage when one is in transition between wakefulness and sleep. The brain waves during this phase are quite rapid and low-voltage. In the beginning of this one does not dream, although photograph like images may appear. Heart rate is elevated and irregular; breathing is rapid and the blood pressure high. Rapid eye movement takes place.
Stage 2 sleep
A level of sleep deeper than stage 1. The wave pattern becomes more regular, that may momentarily show sharp peaks or waves that are sharply pointed called “sleep spindles”. If a person is at stage 2 sleep, it becomes difficult to wake him up.
Stage 3 sleep
The brain waves are slower. Higher peaks and lower valleys are shown by the wave pattern. Stage 3 and stage 4 sleep dominate the first half of the night.
Stage 4 sleep
In this stage the person is almost cut off from the external world and is least responsive to stimulation from outside: the deepest sleep. The brain wave pattern now is more regular and even slower. The time during which this stage is most likely to occur is the early part of the night. Lighter stages of sleep dominate the last half of night.
An electroencephalogram (EEG) is a test that detects electrical activity in your brain using small, flat metal discs (electrodes) attached to your scalp. Your brain cells communicate via electrical impulses and are active all the time, even when you’re asleep. This activity shows up as wavy lines on an EEG recording.
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