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 December 24, 2008, online issue of Neurology®, the medical journal of the American Academy of Neurology.

The sleep disorder is called REM sleep behavior disorder. 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…