Rapid eye movement (REM) sleep is a mentally active period during which dreaming occurs. REM sleep is marked by extensive physiological changes, such as accelerated respiration, increased brain activity, eye movement, and muscle relaxation. REM sleep is a period late in the sleep cycle in which the brain and body become active, increasing heart rate and blood pressure.

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.

Intense dreaming occurs during REM sleep as a result of heightened cerebral activity, but paralysis occurs simultaneously in the major voluntary muscle groups, including the sub-mental muscles like the chin and neck. Because REM is a mixture of brain (encephalic) states of excitement and muscular immobility, it is sometimes called paradoxical sleep. It is generally thought that REM-associated muscle paralysis is meant to keep the body from acting out the dreams that occur during this intensely cerebral stage. The first period of REM typically lasts 10 minutes, with each recurring REM stage lengthening, and the final one lasting an hour.

REM sleep performs many functions among them development of the brain, synthesis of neuro-proteins, and coordination of eye movements. In addition, memories that we don’t think about from day to day are relived during REM sleep, in the form of dreams, so that we can remember them when needed. REM is the mechanism used by the brain to promote recovery from sleep.