Sleep paralysis is the sensation of feeling paralyzed upon awakening, usually immediately following a dream. This is commonly associated with the loss of muscle tone during dreams, called atony. This loss of muscle tone during the dream sleep (REM) stage, prevents one from acting out his or her dreams. It is believed to be one of the instances where sleep and wakefulness are not mutually exclusive. It is believed that during the REM sleep, where there is complete muscle inhibition, suddenly the mind wakes up, but there is still atonia, hence, the person if fully awake but cannot move (paralyzed) for that few seconds (sleep paralysis). This can be a scary experience.