stages of sleep

n. The four distinct patterns of electroencephalographic recordings characteristic of human sleep over the course of a night. Stage 1, the initial stage, is characterized by relaxation and slow, irregular, low-amplitude brain waves at about 4–6 cycles per second. The second stage of sleep is characterized by sleep spindles, which are variable in amplitude bursts at about 14–18 cycles per second. The third stage of sleep is characterized by a mixture of large-amplitude, slow brain waves of about 1–4 cycles per second interspersed with sleep spindles which are similar to those in the second stage. The fourth stage of sleep, called deep sleep, is characterized by highamplitude, slow brain waves without the spindle activity of the third stage of sleep. Many people cycle through the four sleep stages in about 90 minutes and have several such cycles in a night's sleep. Rapid eye movement (REM) sleep, characterized by many rapid eye movements and often dreaming, is sometimes considered a fifth stage of sleep, although it typically occurs during other stages, particularly during the first and second stages in the latter half of a night's sleep and at other times of high fatigue.