Old Hag Syndrome
From my thesis work for the Academy of Art.
Sleep paralysis was once believed to be caused by witches, or hags. The name of the phenomenon comes from the superstitious belief that a witch - or an old hag - sits or "rides" the chest of the victims, rendering them immobile. Although that explanation isn't taken very seriously nowadays, the perplexing and often very frightening nature of the phenomenon leads many people to believe that there are supernatural forces at work - ghosts,aliens, or demons.