In humans, a normal night of sleep is usually characterized by alternating periods of slow-wave sleep (or non-REM sleep) and paradoxical sleep (or REM sleep). These periods, called sleep cycles, are generally repeated 4-6 times during the night. During non-REM sleep, we distinguish three stages of sleep known as N1 (light sleep or stage 1 sleep), N2 (stage 2 sleep) and N3 (deep sleep or stages 3 and 4 sleep) and its scoring is based on precise physiological characteristics including the frequency and amplitude of brain waves recorded during electroencephalography (EEG). REM sleep, on the other hand, is characterized by the appearance of an activity similar to that recorded in wakefulness, hence the name "paradoxical sleep", by muscle atonia (paralysis of the muscles) and rapid eye movements (REM). It is during REM sleep that dreams are the most intense and detailed.
Many differences are observed among individuals with normal sleep. For example, the number of hours required for each person to recover and feel functional the next day is very variable. The same is also true regarding the preference for sleep timing as some persons prefer to sleep at an earlier clocktime (morning types) while others prefer to sleep later (evening types). Sleep also changes depending on age. In this sense, the need for sleep is much more important in children and adolescents, whereas in the elderly, the episode of sleep is often shorter, more fragmented and lighter.
During sleep, there are not only significant changes in brain waves but also changes in muscle activity, hormonal secretion, autonomic nervous system activity, body temperature, and a multitude of metabolic, cellular and molecular changes.
Normal sleep night in a young healthy subject presented in a hypnogram.
NREM sleep stages (N1, N2, N3) are succeed by a REM sleep period (PREM) essentially composed of REM sleep (R).