As if nightmares weren't bad enough, a rare sleep disorder causes people to act out their dreams, sometimes with violent thrashes, kicks and screams.

Such violent dreams may be an early sign of brain disorders down the line, including Parkinson's disease and dementia, according to research published online July 28, 2010, in the journal Neurology.

The results suggest the incipient stages of these neurodegenerative disorders might begin decades before a person, or doctor, knows it.