Besides aliens and UFOs, not sure what to think about those. But seriously, watching these TOP 10 NIGHTMARE INDUCING VIDEOS on youtube is just blurry shadows of someone poking their head around a corner or someone in a costume. I get that people want to believe in the paranormal, but c'mon guy, I've seen b-movie schlock more convincing than the shit out there.
There is a common culture regarding some form of hag or demon causing sleep paralysis and nightmares.
Here's the wiki page I found. Do not look at the picture if you want to sleep well tonight. That "horse" creeps me out.
I also had a fever the other day (vaccine lol) and I got flashing images of a warped face in my mind as I was drifting off to sleep. Your mind screws with you normally, imagine what it does when it's overheating lol
I saw images like that all the time before falling asleep when i had an appartment with a really dark bedroom. Just a constant stream of faces, mostly human ones, but also animals, robots and aliens from time to time. Just constantly warping from one face to the other. Most of them didn't look frightening once i got used to that, some where even outright beautiful. A few times, i'd see other things as well, like geometrical patterns or architecture, but faces where by far the most common.
Yeah, it's like your brain is randomly generating faces. I think the fever made them a bit more frightening than usual though.