Researchers may have identified several factors that put people at risk for a disorder that makes them kick, yell and punch in their sleep.

Smoking, head injuries and pesticide exposure are relatively common among people with REM sleep behavior disorder, a rare but serious problem that has been linked to Parkinson's disease and dementia. As many as 50 percent of people with Parkinson's also have the sleep disorder.

"There's really a tremendous lack of information about this sleep disorder," said Dr. Charles Cantor, medical director at the Penn Sleep Centers. "We don't know much about prevalence. We don't know much about the risk factors that may lead to REM behavior disorder," said Cantor, who was not involved with the new research.

About only 0.5 percent of the population has the disorder, according to the National Sleep Foundation. This might explain why the condition was not even identified in humans until the mid-1980s.

People with the disorder lack the quiet, still appearance that typically is exhibited during REM sleep; instead, they act out their dreams, which are often violent or stressful. Experts have identified the disorder as a possible precursor to several neurodegenerative problems, including dementia and Parkinson's disease (the progressive disorder that affects movement).

Given the connection for many individuals between the sleep disorder and Parkinson's or dementia, researchers hypothesized that the various health problems might also share risk factors. They recruited nearly 700 patients, most of whom were male and in their late 60s, at several sites in 10 countries and found that smoking, head injuries and pesticide use were risk factors for the sleep disorder.

Not all those factors contribute to the onset of Parkinson's and dementia, however.

Indeed, smoking has been found to help ward off Parkinson's disease, suggesting the relationship is not a simple one. Study researcher Dr. Ronald Postuma at the McGill University Health Centre in Montreal said that it might be that smoking doesn't protect a person against Parkinson's but simply helps treat some of its motor symptoms.

Another difference was caffeine: Studies have suggested that people who drink coffee are less likely to develop Parkinson's, but the new study, published online on Wednesday in the journal Neurology, found no connection between coffee drinking and REM sleep behavior disorder.

Postuma's goal, he said, is to continue to probe the possible causes of REM sleep behavior disorder so as to better understand how it may predict neurodegenerative problems later in life. This may help doctors intervene, not just by treating the sleep disorder by prescribing clonezepam or melatonin, but also by helping them identify individuals at risk for future neurological problems. While there is no cure for Parkinson's disease or dementia, physicians can take measures to improve the quality of life for those who have such conditions.

"The central message is that this sleep disorder can present 10 to 20 years before people ever get Parkinson's. It is early enough to intervene that we could make a real difference in their quality of life," Postuma said.

"As we continue to develop tools, having a 10- or 20- or 30-year warning would be helpful," Cantor said. "Because ideally, we might be able to help."