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 (either end of stage one trace) transition to theta waves (middle of stage one trace). Stage 2 includes short bursts of high-frequency brain activity known as sleep spindles (bunched waves near start of middle trace) and large amplitude delta waves that last for only a second, known as K-complexes (towards end of middle trace). 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.