Rapid eye movement sleep behavior disorder or REM behavior disorder (RBD) is a sleep disorder in which people act out their dreams. It involves abnormal behavior during the sleep phase with rapid eye movement (REM) sleep. The major feature of RBD is loss of muscle atonia (i.e., the loss of paralysis) during otherwise intact REM sleep (during which paralysis is not only normal but necessary). The loss of motor inhibition leads to sleep behaviors ranging from simple limb twitches to more complex integrated movements that can be violent or result in injury to either the individual or their bedmates. RBD is a very strong predictor of progression to a synucleinopathy (usually Parkinson's disease or dementia with Lewy bodies). Melatonin is useful in the treatment of RBD. RBD was first described