Rapid eye movement (REM) sleep behavior disorder is a sleep disorder in which you physically act out vivid, often unpleasant dreams with vocal sounds and sudden, often violent arm and leg movements during REM sleep — sometimes called dream-enacting behavior.

You normally don't move during REM sleep, a normal stage of sleep that occurs many times during the night. About 20 percent of your sleep is spent in REM sleep, the usual time for dreaming, which occurs primarily during the second half of the night.

The onset of REM sleep behavior disorder is often sudden, and episodes may occur occasionally or several times a night. The disorder can get worse with time.

REM sleep behavior disorder often may be associated with other neurological conditions, such as Lewy body dementia (also called dementia with Lewy bodies), Parkinson's disease or multiple system atrophy.

July 11, 2014