Ultradian rhythms last fewer than 24 hours and can be found in the pattern of human sleep. This cycle alternates between REM (rapid eye movement) and NREM (non-rapid movement) sleep and consists of five stages. The cycle starts at light sleep, progressing to deep sleep then into REM sleep, where brain waves speed up and dreaming occurs. This repeats itself about every 90 minutes throughout the night and a person can experience up to five complete sleep cycles each night.