Sleep paralysis is a phenomenon in which people, either when falling asleep or wakening, temporarily experience an inability to move. More formally, it is a transition state between wakefulnessand 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 (e.g., 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.
The two major classifications of sleep paralysis are isolated sleep paralysis (ISP) and recurrent isolated sleep paralysis (RISP). Of these two types, ISP is much more common than RISP. ISP episodes are infrequent and of short duration, approximately one minute. Sleep paralysis might even only occur once in an individual’s lifetime. As the name suggests, recurrent isolated sleep paralysis is a chronic condition. The individual suffers from frequent episodes throughout their lifetime. One of the major differences between ISP and RISP is duration. RISP episodes can last for up to an hour or longer, and have a much higher occurrence of perceived out of body experiences—while ISP episodes are generally short (usually no longer than one minute) and are typically associated with the intruder and incubus hallucinations. ISP episodes can, however, persist for up to half an hour. With RISP the individual can also suffer back to back episodes of sleep paralysis in the same night while this is unlikely in individuals who suffer from ISP.
It can be difficult to differentiate between cataplexy brought on by narcolepsy and true sleep paralysis, because the two phenomena are physically indistinguishable. The best way to differentiate between the two is to note when the attacks occur most often. Narcolepsy attacks are more common when the individual is falling asleep; ISP and RISP attacks are more common on awakening.