Several of you mentioned the "Old Hag" syndrome, and not to discount your analysis, this is very similar to the phenomenon of sleep paralysis.
Every time it happens to me, it feels like I am asleep in my bed (which I am), but there is something else in my room whose physical presence
I can feel. At it's most benign, it's like a person sitting down on the mattress next to me. Other times it is like a person crawling on top of me and pressing down. Nobody wants that from anybody other than a significant other. And still other times it is like there is a cat in my bedroom who is bouncing around all over my mattress.
Every time it happens it's disconcerting as all get out.
But as scary as that is, this is not an uncommon occurrence in the population at all. It's just an imbalance in the way your body tries to wake you up and the way dream cycles occur. Nothing supernatural, it's just the way your brain can occasionally misfire. It happens to a lot of people, even if they don't all talk about it.
Here's the thing, it only happens to me after I have spent several nights drinking and then quit. Drinking alcohol, if you don't know, messes with your bodies normal REM cycles. Simply put, you don't get the full good nights of sleep you need. If you do that for several days, you back up on REM sleep. Lack of REM sleep has been known to cause full blown psychosis in conscious
individuals. So if you drink multiple nights in a row, and then stop, your body has a bunch of neurotransmitters it's been waiting to put back in to good use, which it does.
And most times I do that I get sleep paralysis.
Now you could blame that on actual demons, but in my case its more on my purposeful use of "demon drink" messing with my biochemistry. It is the only time it ever happens to me.
Don't rule it out. I'd be happy to discuss the role of neurotransmitters and sleep cycles if you read the article and don't close your mind.