For most people, dreaming is purely a "mental" activity: they occur in the mind while the body is at rest. But people who suffer from REM behavior disorder (RBD) act out their dreams. They physically move limbs or even get up and engage in activities associated with waking. Some engage in sleep talking, shouting, screaming, hittting or punching. Some even fly out of bed while sleeping! RBD is usually noticed when it causes danger to the sleeping person, their bed partner, or others they encounter. Sometimes ill effects such as injury to self or bed partner sustained while asleep trigger a diagnosis of RBD. The good news is that RBD can usually be treated successfully.
What we call "sleep" involves transitions between three different states: wakefulness, rapid eye movement (REM) sleep, which is associated with dreaming, and non rapid eye movement (N-REM) sleep. There are a variety of characteristics that define each state, but to understand REM Behavior Disorder it is important to know that it occurs during REM sleep. During this state, the electrical activity of the brain, as recorded by an electroencephalogram, looks similar to the electrical activity that occurs during waking. Although neurons in the brain during REM sleep are functioning much as they do during waking, REM sleep is also characterized by temporary muscle paralysis.
In some sleep disorders such as narcolepsy and parasomnias, like REM behavior disorder, the distinctions between these different states breaks down; characteristics of one state carry over or "invade" the others. Sleep researchers believe that neurological "barriers" that separate the states don't function properly, though the cause of such occurrences is not entirely understood.
Thus, for most people, even when they are having vivid dreams in which they imagine they are active, their bodies are still. But, persons with RBD lack this muscle paralysis, which permits them to act out dramatic and/or violent dreams during the REM stage of sleep. Sometimes they begin by talking, twitching and jerking during dreaming for years before they fully act out their REM dreams.
In the course of "acting out their dreams," people with RBD move their arms and legs in bed or talk in their sleep, or they might start sleepwalking without waking or realizing they're dreaming. The only sensations the sleeper experiences are what is occurring in their dream. And many of these dreams can be violent or frightening, causing injury to the sleeper and his bed partner.