How long is a sleep cycle?

Each cycle of sleep is approximately 90 minutes, according to Psychology Today. During each 90-minute cycle, the body goes through different stages of sleep. A person goes through four to six sleep cycles in an average night, according to HelpGuide .

As each sleep cycle occurs, there are two types of sleep that a person will experience. Non-rapid eye movement, or NREM, sleep is longer in duration than rapid eye movement, or REM, sleep during the first sleep cycle. With each consecutive sleep cycle, the NREM portion of the sleep cycle decreases in duration and the REM portion of the sleep cycle increases. The deepest sleep comes during the fourth stage of NREM sleep.