REM sleep behavior disorder

REM behavior disorder is a little-known sleep disorder, although it is relatively common (about 1 in 200 people). In a normal person, REM (rapid eye movement) sleep is characterized by theta waves on an EEG, atonia (loss of muscle tone), and dreaming. However, the muscles in an RBD patient are not properly inhibited, and they act out their dreams. For instance, a person that dreams he is chasing after someone will actually pedal in his sleep.

Although the following video cannot be embedded, I recommend checking it out for a basic understanding of the disorder, as well as its potential negative consequences when not properly diagnosed:

ABC Good Morning America - 'Sleep Disorder Puts Husband in Jail'

RBD is believed to be caused by neuronal cell loss in the area of the brain that suppresses muscle tone. It tends to be more common in older people, and much more common in men. However, these estimates are skewed by the fact that people often don't know that they have RBD until they have a bed partner that witnesses their behavior.

The Chicago Tribune - Sleep Disorders a Nightmare for Many