Mahowald and Schenck have been studying a parasomnia that strikes during a different stage of sleep -- REM (rapid eye movement) sleep, when most dreaming occurs. What they found, Mahowald says, is a fascinating link to neurological illnesses such as Parkinson's disease.
Normally during REM sleep, the muscles are completely paralyzed, "so if you dream you are killing your mother-in-law, you're not actually going to do it," Guilleminault says. But people with the parasomnia dubbed REM sleep behavior disorder have an abnormality that causes the normal paralysis of REM sleep to fail.
The result: Victims act out their dreams -- thrashing, swearing, punching, kicking, running out of bed, even pummeling their bed partners, Mahowald says.
Of the dozens of otherwise healthy people with REM sleep behavior disorder that Mahowald and Schenk have followed since the 1980s, two-thirds have gone on to develop Parkinson's disease or other related neurodegenerative disorders, Mahowald says. Most are men, over 50 years old, with the average time between the development of the sleep problem and the neurological disorder being 13 years.
Taking the research a step further, other scientists have implicated the same faulty brain chemistry in both disorders. In a recent study of 13 people with the condition and 27 healthy individuals, Mayo Clinic researchers found REM sleep behavior disorder is associated with low stores of dopamine -- the same neurotransmitter known to be deficient in Parkinson disease. The greater the loss of dopamine in the brain, the more severe the symptoms, the researchers reported in the journal Neurology.
Other researchers have done brain imaging scans of people with REM sleep behavior disorder. They found abnormalities in the region of the midbrain where Parkinson's originates -- even in patients who did not yet have signs of neurological problems.
Though the work is still early, it suggests that REM sleep behavior disorder can be the first symptom of Parkinson's disease, Mahowald says. "If we can develop a drug that protects against Parkinson's, this will be very important."
Sleep Sex, Sleep Eating
Less well known and more recently recognized are sleep sex and sleep eating disorder. "They have some characteristics of each of the other [parasomnias], but don't quite fit the mold," Shapiro says.