There's a fascinating article on Science News about a phenomenon that appears remarkably common, although not widely known -- that of sleep paralysis. Sleep paralysis is a dreaming state quite distinct from night terrors, or nocturnal panic, or even nightmares (although apparently this is the origin of our word for nightmares -- the word has changed its meaning over the centuries). Sleep paralysis is associated with dreams of evil spirits, monsters, demons, aliens ... often crushing your chest, or simply standing ominously nearby (or performing experiments on you!) The phenomenon is often widely acknowledged in pre-industrialized cultures (where presumably it is acceptable to talk about dead or evil spirits visiting you), but not in modern Western society. However, sleep paralysis is now becoming of scientific interest, probably because of its surprising frequency -- in surveys, roughly 30% have claimed to have had at least one such experience.