REM behavior disorder

A parasomnia in which the muscle paralysis which normally accompanies REM sleep does not take effect, causing the subject to act out his or her dream.  REM behavior disorder is physically dangerous to the victims and their bed fellows, as their actions tend to be violent.  Middle-aged men are more likely than any other group to suffer from the disorder, which leads scientists to hypothesize that there is a hormonal component to REM behavior disorder.  Another theory holds that the disorder is linked to a problem in the brain stem, due to atrophy or age. 

Source:

Dement, The Promise of Sleep