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Outline of Sleep Physiology

Sleep cycle
When the normal people are sleeping, besides of the brain activities, the body temperature, blood pressure, respiration, heart rate and all physiological activities will reduce. If we monitor the brain neuronal activities with the brain waves during sleep, then we will see that it presents periodical cyclic activities during the sleep, and the human sleep periodical phase is a cycle of 90-120 minutes, in one night, there are about 4-5 sleep cycles.

Besides of the periodical activities of brain waves, the human sleep cycles can be divided into Rapid Eye Movement (REM) and Non Rapid Eye Movement, (NREM) phase according to the activity of the eyeballs. Where the Non Rapid Eye Movement (NREM) Sleep is about 75% of the total sleep time, and it is divided in 4 stages: the first and second stages are what we know as “light sleep”, the third and the fourth stages are the “deep sleep”. The Rapid Eye Movement (REM) Sleep typically occupies 25% of total sleep, its characteristics are: horizontal rapid eyeball movements, low muscle tone, irregular breathing rate, reduction of body temperature, changes of heart rate and blood pressure.

When the normal adult sleeps, the sleep mostly goes into the first stage, and then goes more deeply while sleeping, the frequency of the brain waves slows down, then it goes into the stage 2, stage 3, even stage 4 and the Rapid Eye Movement (REM) Sleep. While the sleep goes from light deep to deeper sleep, the muscle tone will also be reduced. Normally at the Rapid Eye Movement (REM) Sleep, the muscle tone of the four extremities is at the lowest. Our sleep does not necessarily follow the order of stages 1, 2, 3, and 4, sometimes the sleep starts from the stage 1, and goes into the stage 2 and then directly to the Rapid Eye Movement (REM) Sleep.

Fig.1. Normal sleep cycles