Medscape has an article from the Movement Disorders Society 14th International Congress on Parkinson's Disease and Movement Disorders. In the article, it describes research linking falling out of bed while asleep and Parkinson's Disease.
I see this occur in my practice too, and I'll explain what is happening. Normally, our bodies are temporarily paralyzed in dream sleep, so that we don't act out our dreams while asleep. In patients with Parkinson's or Parkinson's-like disorders, that dream paralysis mechanism doesn't always work so well. If this occurs, patients can act out their dreams, with screaming, arms waving, or even attempting to get out of bed. Since patients are responding to their dreams, they often fall right after getting out of bed, sometimes injuring themselves. This problem is referred to as REM sleep behavior disorder.
This might sound ironic, as Parkinson's disease patient's have difficulty moving while awake, but could be moving too much while asleep. Sometimes, this can lead to daytime sleepiness. Unfortunately, Parkinson's disease itself and the medications used to treat Parkinson's disease can lead to daytime sleepiness.
Just to complicate matters more, REM sleep behavior disorder can be worsened by untreated obstructive sleep apnea. Certain medications can worsen the movements in dream sleep as well. So, if a patient is moving too much in their sleep, the exact cause can be difficult to figure out.
Fortunately, REM sleep behavior disorder is usually treatable with medications and making the bedroom environment as safe as possible when the patient sleeps.