Scientists discovered patients with a violent rapid eye movement sleep behavior disorder, or RBD, have a high probability of developing Lewy body dementia, Parkinson's disease or multiple system atrophy. Lewy bodies are abnormal aggregates of protein that develop inside nerve cells

"Our data suggest many patients with idiopathic RBD may be exhibiting early signs of an evolving neurodegenerative disease, which in most cases appear to be caused by some mishap of the synuclein protein," said Mayo Clinic Dr. Bradley Boeve, the lead author of the study.

Just what happens in the protein following gene expression isn't clear, said Boeve. But he said the result is quite clear: patients -- usually older males -- strike out violently, often yelling, when they enter the rapid eye movement stage of sleep. Many such patients develop symptoms of dementia and postmortems show they all develop Lewy bodies.

The study, which included researchers from Harvard Medical School, the University of Minnesota, and Germany's Goethe University, is detailed in the journal Brain.