During sleep, a person goes through four to five REM periods. REM are rapid eye movement states that last 30 to 90 minutes. During REM periods, brain waves detected via electroencephalography are similar to those of an awake person. In addition to REM periods, sleeping people go through periods of NREM, non-rapid eye movements. NREM periods occur between REM periods and last as long as REM periods. Dreams most commonly occur during REM cycles due to increased brain wave activity. Dreams can occur during periods of NREM sleep, but they are not as common and tend to be less visual. On average, a person has a dream once in three nights, regardless of whether he is able to remember it or not. Dreams are more common among children and gradually decrease as they age