Sleep paralysis or night terrors are as old as mankind, and form a large section of traditional folklore and bard traditions.
My favorite is from "1001" Arabian nights, were the degeni sets on the end of the bed and flips beans at the dreamer.
The Norse Mermen are another, as well as the Old Hag stories, (witches)
most religions encourage the relating of these in their public meetings to help their members through the experience, or other motives
The experience can be invoked sometimes by giving (not counting Hallucinogens) beta blockers, which suggest that (getting away from the psychologists terminology, who think they own the definitions) that the Circadian rhythm has started the cortisol pathway, and their isn't enough epy to "wake" the dreamer.
these trance like states can also be mimicked by low thyroid.
