Researchers have found that REM(rapid eye movement)sleep is important to human beings. This type of sleep generally occurs four or five times during one night of sleep lasting five minutes to forty minutes for each occurrence. The deeper a persons sleep becomes, the longer the periods of rapid eye movement. There are physical changes in the body to show that a person has changed from NREM(non-rapid eye movement)to REM sleep. Breathing becomes faster, the heart rate increases, and, as the name implies, the eyes begin to move quickly. Accompanying these physical changes in the body there is a very important characteristic of REM sleep. It is during REM sleep that a person dreams. According to the passage, how often does REM sleep occur in one night?