WASHINGTON, Dec. 25 (Xinhua) -- People with a sleep disorder that causes them to kick or cry out during their sleep may be at greater risk of developing dementia or Parkinson's disease, according to a study published in the Dec. 24 online issue of U.S. journal Neurology.

The sleep disorder is called REM sleep behaviordisorder. People with the disorder do not have the normal lack of muscle tone that occurs during REM sleep, often known as the dream stage of sleep. Instead, they have excessive muscle activity such as punching, kicking, or crying out, essentially acting out their dreams.

The study involved 93 people with this type of sleep disorder who had no signs of a neurodegenerative disease, such as dementia or Parkinson's disease. The participants were followed for an average of five years.

During that time, 26 of the people developed a neurodegenerative disease. Fourteen developed Parkinson's disease,11 developed dementia and were diagnosed with either Alzheimer's disease or Lewy body dementia. One person developed multiple system atrophy, a rare disorder that affects movement, blood pressure and other body functions.

The estimated five-year risk of developing a neurodegenerative disease was 18 percent, with the 10-year risk at 41 percent and the 12-year risk at 52 percent.

"These results are obviously of great interest to people who have this sleep disorder and their physicians and families," said study author Ronald Postuma from McGill University in Canada. "The results may help us better understand how these neurodegenerative diseases develop. They also suggest that there may be an opportunity for protecting against the progression to disease, perhaps even preventing it before the symptoms can appear."