A person who is deprived of stages three and four becomes apathetic and depressed. This could also lead to fibrositis. REM sleep starts after about 90 minutes of sleep. The person gradually returns to lighter stages of sleep. REM and non-REM sleep alternate at 90 minute intervals four or five times a night.
Most dreams occur during REM sleep.
In infants, REM sleep is important for psychological adaptation, integrating daily activities with memory and learning. If a person does not get enough of this type of sleep, it may cause irritability, apathy, fatigue, depression, loss of concentration and judgement. He or she may also become more sensitive to pain and discomfort. Natural cortisone is released in the body eight times in a 24-hour cycle. Five of these releases occur during REM sleep in the early morning hours.