Our sleep move in cycles, first it moves from wakefulness then into a deep phase, regenerative sleep, then back to alertness and again descends. Sleeping normally varies between 4 and 6 such cycles at night, each of which lasts between 70 and 110 min.
During the deepest phases of the sleep in the brain there are waves that come slowly and regularly, also blood pressure, temperature and muscle tone are decreased.
These are the steps:
Step 1 ( Drowsiness )
Going in and out from dreaming about 5-10 minutes. And at this point, you can easily be awakened. Our eyes move very slowly and muscle activity is weak.
Stage 2 ( light sleep )
The eye movements stop and our brain waves ( fluctuations of electrical activity that can be detected with electrodes) are delayed by occasional bursts of rapid waves. Our heart rate is slowed and the body temperature is lowered.
Stage 3 and 4 ( deep sleep )
Slow brain waves called delta waves begin to appear, scattering with smaller, faster waves. By stage 4 brain produces delta waves almost exclusively. It is very difficult to wake someone during phase 3 and 4, which together are called deep sleep. No eye movement and muscle activity. People who were awakened during deep sleep often feel tired and disoriented for a few minutes until awakened.
REM sleep or ( Rapid eye movement sleep)
during REM sleep our breathing becomes more rapid, irregular and shallow; our eyes flicker rapidly in different directions, and our muscles temporarily are paralyzed. Our heart rate increases, blood pressure and also men get an erection at this point. People tend to have clear dreams during this phase of sleep.
Children are unique in that they spend more than 50% of their sleep in the REM stage
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