Sleep is not a unique entity but can be divided into 2 separate states called REM (rapid eye movement) and nonREM sleep stages. The present study describes REM sleep in the unrestrained chimpanzee and compares the first REM period which occurs during the night with the last REM period. The last REM period is longer, has more eye movements, more body movements, and possibly a different type of posture than early REM periods. These differences suggest that REM sleep, as is already known for nonREM sleep, changes during the course of the night.