You've probably seen movies where a veteran returns home from the horrors of war and wakes in the middle of the night yelling, punching or flailing so much that they harm themselves or a sleep partner.

This isn't just Hollywood drama. New research has identified who's most at risk for this troubling sleep condition.

It's called rapid eye movement (REM) sleep behavior disorder, and about 1 percent of people over 50 who have a sleep study are diagnosed with it.

"REM sleep behavior disorder is different than sleepwalking. It only occurs during REM sleep, when we dream," explained the study's senior author, Dr. Ronald Postuma. He's a professor of neurology at Montreal General Hospital in Montreal, Quebec.

Normally, people are temporarily paralyzed during REM sleep. This usually keeps people from acting out their dreams. But in people with REM sleep behavior disorder, something goes wrong with the system that paralyzes people during sleep, Postuma said.

"Most people think this behavior is normal," he said, adding that people often don't ask their doctor about their bizarre sleep behavior until they hurt someone.

People can become quite violent. Some have even used a diagnosis of a sleep disorder such as REM sleep behavior disorder to explain away violent crimes in court, according to Postuma and published reports.

About 80 percent of REM sleep behavior disorder patients will develop a very serious neurological condition, such as Parkinson's disease or a type of memory and thinking condition called dementia with Lewy bodies, according to the study. REM sleep behavior disorder may be an early marker of trouble in the body's neurological system, Postuma noted.

To identify other risk factors linked to REM sleep behavior disorder, researchers asked more than 30,000 participants in a long-term study on aging in Canada if they had ever been told or suspected themselves that they had acted out their dreams while asleep.

Almost 1,000 people said yes.

The investigators found that people who had REM sleep behavior disorder were twice as likely as the general public to have mental illness, and more than 2.5 times as likely to have post-traumatic stress disorder. In addition, these people were more than 1.5 times more likely to report having psychological distress.

Study participants with REM sleep behavior disorder were also far more likely to be taking antidepressants -- 13 percent of those with the sleep condition took antidepressants compared to 6 percent of others, the study found.

Men were more likely to have the condition. Those with the sleep disorder were also more likely to be moderate to heavy drinkers.

Postuma said it's hard to know if any of these risk factors causes the disorder or if they simply help to unmask a problem that's already there.

He said it's important not to "overinterpret the study's findings."

Dr. Thomas Kilkenny, director of sleep medicine at Staten Island University Hospital in New York City, noted that the study cannot prove a cause-and-effect link between any of the risk factors and REM sleep behavior disorder. Still, he said that "the article demonstrates strong associations."

Kilkenny said a limitation of the study is that it relied on information from patients themselves. "However, the sheer large volume of patients evaluated lend strength to the reporting," he added.

Postuma pointed out that identifying people with REM sleep behavior disorder will likely become even more important if a preventive medication for Parkinson's disease is developed. He said this group could be identified before Parkinson's symptoms appear, making them ideal for a preventive medication clinical trial.

But for right now, if you act out during your sleep or have a partner who does, Postuma has some advice for you: "If you're an older person acting out your dreams at night, that's not normal. Go see your doctor."

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