Ordinarily, the body lies virtually paralyzed during REM (rapid eye movement) sleep, which is the phase of sleep in which we tend to dream, but in people with this condition the normal paralysis of REM sleep fails and patients act out their dreams, frequently injuring themselves or their bed partners.

One man dreamed his wife was a wild animal and tried to choke her. Another walked through a plate glass door, severing an artery. Most who suffer from this ailment are men over fifty years of age.

It typically appears at least 90 minutes after sleep onset and about once a week but may appear as frequently as four times per night during several consecutive nights. An acute, transient form may accompany REM rebound during withdrawal from alcohol and sedative-hypnotic agents. Drug-induced cases have been reported during treatment with tricyclic antidepressants and biperiden.

More dominant in males than in females, the disorder usually begins in late adulthood (sixth or seventh decade), progresses over a variable period of time (months to years), and then may stabilize. Approximately 60 percent of cases are idiopathic and the remaining cases are associated with neurological disorders such as dementia, subarachnoid hemorrhage, isclemic cerebrovascular disease, olivo-ponto-cerebellar degeneration, multiple sclerosis, and brain stem neoplasm. Family history patterns are suggested but insufficient information is available.