Sleep paralysis is a phenomenon in which a person, either when falling asleep or awakening, temporarily experiences an inability to move. It is a transitional state between wakefulness and sleep characterized by complete muscle atonia (muscle weakness). It is often associated with terrifying visions, such as an intruder in the room, to which one is unable to react due to paralysis, from which the term "nightmare" is derived. One theory is that it results from disrupted REM sleep, which is normally characterized by complete muscle atonia to prevent the sleeper from acting out his or her dreams.