There have been reports of sleeping persons being attacked in their slumber by hideous beasts for centuries. We are referring here not to jackals or bears or wolves but of demons, shape-shifters, aliens, and alluring women whose beauty masks an intent most evil.
The tales vary by culture, but as Brian Dunning noted in a Skeptoid podcast, the stories within a culture are largely uniform.
In Europe, especially in and around Shakespeare’s time, the attackers were sometimes thought to be incubus and succubus, demons who raped their victims. This was consistent with a time that blamed every misfortune on the devil. More frequently, the creeping creature took the form of a Macbeth-worthy witch. This could play on the aversion and distrust of old maids, who were chastised for shirking their societal responsibility to have and raise children.
In the East, the female fiend is the physical opposite of an old hag, and is portrayed as a stunningly beautiful. But the intent remains the same, with the nocturnal interloper using charm and guile, rather than force and revulsion, to overtake her victim.
Again, this plays on society’s prejudices, treating women as temptresses leading individuals and civilization to ruin.
One culture that bypasses the prejudices are the Slavs, whose nighttime attackers were described in a Brian Dunning Skeptoid podcast as “an elf-like gypsy man with wild glowing eyes who sits on your chest, riding you like a horse.” Meanwhile in Japan and China, these apparitions have traditionally taken the form of ghosts.
Consistent with the notion of attackers adapting to the times, the nightmarish visions turned to alien visitors with the advent of the Space Age. Witches’ cackles were replaced with silent telepathic communication and clubs were transformed into probing implements.
While there have been many varieties of nighttime attackers, a common thread is that these are likely the result of sleep paralysis. That would explain most of what happens, with cultural expectations being responsible for some specific details.
During sleep paralysis, the victim is unable to move or speak and often feels as if their chest is being crushed. This may be accompanied by hallucinations. Dunning noted that paralysis only happens during REM sleep and this makes sense, as frightful characters would manifest in dreams.
Sleep paralysis occurs because of a disconnection between the brain and the rest of the body when drifting into or out of sleep. And since REM represents the deepest stage of sleep, this is when the disconnection is at its most pronounced.
Sleep paralysis is a reasonable explanation for persons thinking they’ve been assaulted by witches, sirens, elves, and aliens. The presence victims feel would be consistent with hallucinations, a sleep paralysis hallmark.