North American Prodromal Synucleinopathy Consortium Stage 2

This study is following people who have REM sleep behavior disorder — a condition where people physically act out their dreams while sleeping — because this condition can be an early warning sign of Parkinson's disease or a related brain disorder. The study also enrolls healthy matched controls for comparison. **You may be eligible if...** - You are over 18 years old - You have been diagnosed with REM sleep behavior disorder (confirmed by a sleep study) - OR you are a healthy volunteer matched by age, sex, and race to someone with REM sleep behavior disorder - You are willing to undergo brain imaging and a lumbar puncture (spinal fluid test) **You may NOT be eligible if...** - You already have a diagnosis of Parkinson's disease, dementia, or multiple system atrophy (unless you previously participated in the first stage of this study) - You have narcolepsy - You have an active psychiatric or medical condition that would make participation unsafe - You have a contraindication to MRI or lumbar puncture Talk to your doctor to see if this trial is right for you.