The experience, after going to sleep, of awakening (or believing yourself to be awake) and not being able to move. Sometimes frightening, this experience has been associated with paranormal phenomena, including ghosts, demons, etc. However, according to surveys, this may be a natural phenomenon with as many as 60% of respondents claiming to have had this experience. Because predicting when it will occur is not possible, an accepted cause or set of causes for this phenomenon has not been determined.