This is an old but new topic that has not yet been satisfactorily answered. In the early days, it was thought to be a kind of suspension of consciousness, and later it was regarded as a temporary mental rest, similar to animal hibernation, a kind of nerve cell “disconnection and deactivation”, and only since the availability of electrophysiological technology, people gradually unveiled its “mystery”. Since the advent of electrophysiological technology, people have gradually uncovered its “mystery”. It turns out that human sleep is not a passive, static process as originally thought, but simply due to the continuous entry of various stimuli into the brain during fatigue or awakening. Polysomnography reveals that human sleep is an active physiological process with periodic patterns in several different phases, which is roughly divided into rapid eye movement (REM) and non-rapid eye movement (NREM) sleep according to the characteristics of EEG, oculogram and electromyogram. In the REM phase sleep, rapid and irregular reciprocal eye movements, accelerated respiration and heart rate, increased blood pressure, and decreased muscle tone can be found; while non-REM sleep can be further divided into four periods: 1, 2, 3, and 4, during which regular and characteristic EEG activity such as sleep spindle waves and high amplitude slower EEG activity can be seen. One REM sleep and non-REM sleep constitute one sleep cycle of about 60-90 minutes, and about 4-7 such cycles can be experienced in one night, with REM sleep accounting for about 20-25% and non-REM sleep accounting for about 75-80% of the overnight sleep of young people. Further studies have found that in the REM sleep phase, continuous sleep conditions produce brief periodic awakenings, which help the body to remain alert to changes in the surrounding environment, while increasing protein synthesis in infants and children, and in adults, which help to maintain the integrity of neurological functions and participate in the transformation and consolidation of memory. In the non-rapid eye movement phase of sleep, it is found that the energy consumption of the organism decreases, the secretion of various prohormones from the pituitary gland increases, and the secretion of growth hormone reaches its peak in phase 4, thus eliminating the fatigue of the organism and restoring physical strength. In conclusion, sleep is very important for human physiological and psychological health, people should arrange their lives scientifically, work and rest regularly, have good sleep and enjoy a happy life.