Your sleep cycle
Scientists use electroencephalography (EEG) to measure electrical brain activity during the five stages of sleep. When you fall asleep, you pass through stages 1 to 4 (the deepest level of sleep) as your brain becomes less active. You then go through the stages in reverse before 5 to 15 minutes of stage 5: rapid eye movement (REM) sleep. During REM sleep, your eyes dart about, your brain is frantically active and you dream. The whole cycle lasts about 90 minutes and repeats throughout the night.