Sleep paralysis is a phenomenon in which people, either when falling asleep or wakening, temporarily experience an inability to move. It is a transition state between wakefulness and rest characterized by complete muscle atonia (muscle weakness). It can occur at sleep onset or upon awakening, and it is often associated with terrifying visions, such as an intruder in the room, to which one is unable to react due to paralysis. It is believed to be a result of disrupted REM sleep, which is normally characterized by complete muscle atonia that prevents individuals from acting out their dreams. Sleep paralysis has been linked to disorders such as narcolepsy, migraines, anxiety disorders, and obstructive sleep apnea; however, it can also occur in isolation. When linked to another disorder, sleep paralysis commonly occurs in association with the neurological sleep disorder narcolepsy.