Picture: René Magritte Elaboration: S.H. Cardoso

You must have perceived someone rapidly moving his or her eyes while this person was asleep. This process is called the "Rapid Eye Movement" sleep, or REM sleep, a mentally active period during which dreams occur. Many times during the night you enter in the REM sleep. While in this stage of the sleep, many researchers awakened voluntary subjects for the research, and observed that they could recall dreams. In other stages of sleep (conventionally called Non-REM (NREM) sleep, the subjects didn't remember if they have dreamt or not (3). In the NREM sleep the brain generally doesn't generate dreams; it seems to be meant to rest.

The eye movements generally happen in the horizontal way and it seems to represent a laborious scan of the action scene in the dream. In rare occasions, researchers have observed that rapid eye movements were vertical, and in this situation, when the persons were awakened, they related dreams involving up and down movements of objects or persons (3). In another situation, when the register didn't show any eye movement and the EEG indicated a dream, the subjects mentioned they were observing some distant point in their dreams. Thus, it seems the direction of eye movements corresponds to what the dreamer is looking or following with his or her eyes.

In the REM sleep, the individual seems to be more awaken than asleep, however the body (except for the eye muscles) is immobilized.

However, it is important to mention that REM sleep and dreams are not synonimous. Some dreams - although rare - may occur out of a REM period. Furthemore, REM sleep has some peculiar characteristics which are not always associated to dreaming.

Author: Silvia Helena Cardoso, PhD. Psychobiologist, master and doctor in Sciences by the University of São Paulo and post doctoral fellowship by the University of California, Los Angeles. Invited Professor and Associate Researcher of the Center for Biomedical Inofrmatics, State University of Campinas (Unicamp), Brazil. Email:

Translation: Sirley Marques Bonham, Email:sbonham@io.com