Illustration of electroencephalogram (EEG) traces while awake (top) and in the various stages of sleep. The first stages are all non-rapid eye movement (NREM) sleep. During stage one, as the person is falling asleep, alpha brain waves transition to theta waves. Stage 2 includes short bursts of high-frequency brain activity known as sleep spindles and large amplitude delta waves that last for only a second, known as K-complexes. Stage 3, which can be subdivided into two stages, is deep slow-wave sleep that is dominated by delta waves. NREM sleep is followed by a period of REM sleep (bottom), during which most dreaming occurs. It takes on average 90 minutes to complete all the stages of sleep, which are cycled through several times a night.