Dream, mental activity that occurs during sleep. Dreaming occurs during the REM (rapid-eye movement) period of sleep. Dreams are often quite vivid, though research indicates that most people forget the majority of their dreams. The interpretation of dreams has played an important part in pre-modern cultures, and major theories of dreams and their significance have been put forward by Sigmund Freud, Carl Jung, and others. In the latter 20th century, scientific thinking about dreams has focused on research into the physiological characteristics and mechanisms of sleep and the activity of the brain while in the sleeping state.