According to Professor Chris French on The Guardian, sleep paralysis is thought to be at the root of stories about nocturnal encounters with witches, succubi and demons, and reported instances appear all over the world in various folklore. In Newfoundland, myths about an old hag abound, while in China and Taiwan, sleep paralysis is reported as "ghost oppression." In more modern parlance, alien abductions can be attributed to sleep paralysis.
Sleep paralysis occurs when the normal sleep cycle is interrupted and the mind becomes aware of the body's paralysis, which occurs during REM sleep. Associated symptoms include the strong sense of an additional presence, difficulty breathing due to a weight on the chest, fear and visual, auditory, tactile and proprioceptive hallucinations. Basic sleep paralysis is relatively common and a symptom of narcolepsy, with one in 20 reporting experiences of the associated symptoms.
Not all people report supernatural interpretations of their sleep paralysis symptoms. Some instead believe that their homes have been broken into by a murderer, burglar or rapist, suggesting that the fear accompanying sleep paralysis is triggered by the amygdala rather than as a consequence of the hallucination. Unlike lucid dreaming, sleep paralysis cannot be controlled despite awareness. Professor French attributes most reports of paranormal or supernatural encounters to sleep paralysis.