Much of what we know about sleep and its effects comes from electroencephalography studies, a technique based on applying electrodes to the scalp to record brain electrical activity . The recording of this activity, which is manifested by brain waves, is known as electroencephalogram ( EEG ). Using this technique, the researchers identified two distinct stages of sleep, each characterized by specific physiological changes: slow-wave sleep and REM sleep (from the English " Rapid Eye Movement ").
Slow wave sleep or non-REM sleep (NREM)
Non-REM sleep accounts for about 75-80% of total rest time in adult subjects; consists of 4 stages characterized by an increase in sleep depth.
During slow-wave sleep, the brain emits low-frequency electrical waves (in wakefulness, brain waves are fast and irregular). Muscle tone is decreased, but occasional involuntary motor activities are recorded (when, for example, one changes position in the bed). Pressure, temperature and heart rate also decrease.
During slow-wave sleep a person can think and dream, but thoughts are more logical and with less emotional content than those that occur during REM sleep. Dreams are not very detailed and often include sensations and vague images.
REM sleep
REM sleep follows each NREM sleep cycle (note: sleep phases do not occur only once a night, but alternate several times for a total of 5-6 complete cycles lasting 90-100 minutes). This stage is characterized by high-frequency electroencephalographic waves and episodes of rapid eye movements below closed eyelids . The postural muscles lose tone, however the muscles that control the face, the eyes and the distal portion of the legs frequently become active in a phased manner.
REM sleep corresponds to about 20% of the total rest time. Compared to slow wave sleep, there is an increase in brain activity in this period except in the limbic system, where, on the contrary, neuronal activity decreases. It is at this stage that dream activity occurs: dreams are more elaborate and intense and are generally very complex. In REM sleep, thoughts are more illogical and bizarre than slow wave sleep.
