Sleep paralysis is very interesting.
You should read about it on Wikipedia. Sleep paralysis is the strange sensation of being awake while dreaming, unable to move, and often accompanied with the belief that there is someone else standing over you, possibly holding you down.
You can attribute pretty much all stories about alien abduction and ghostly visits to the natural occurrence of sleep paralysis.
I’ve experienced a handful of incidents over the past few years. Once, immediately after my cat died, I felt a warm presence pressing down on my chest while I slept. If I were a different sort of person, I would have thought, “Oh me oh my, my dead cat is visiting me from beyond the grave!” In my dream state that is what I surmised, but as I came closer to consciousness, I realized it was all in my imagination. I was oddly paralyzed by this warm force, but I knew it wasn’t real.
Other times there are nightmares, like the legends of yore. It has been very rare for me. However, there are few things as terrifying as waking in the dim blue light of the deepest night under the sensation that some dark force is holding you down against your will. The feeling is definite, immediate, and real. When you wake, you search desperately around your bed to assure yourself that you are alone but are never convinced, until you once again descend into slumber. According to this Guardian article, many people seem to talk about the presence trying to suffocate them. There is hardly anything more terrifying than your own imagination.
Sometimes in my half-conscious state I’ve felt it creeping up and managed to wiggle away from it, shifting my position in bed like some telekinetic act requiring great force of will. It’s a strange thing. If I feel it coming on, I try to avoid it because of the potential nightmares. Most recently I recall feeling dark bands of cold winter air, sentient wisps of winter wind, holding me down in my bed in my drafty, chilly apartment. Maybe that was a normal dream.
I also dream frequently about plane crashes. Oddly, they aren’t nightmares! But that is a story for another day.