Sleep Paralysis is when the brain wakes up but the body is still in sleep mode.
You wake up & become aware of your surroundings but you can't move or speak. It is common for most to experience this, though some might not remember or think they had it but thought it was a dream.
If you have halluncinations when this occurs & feels like your chest is being compressed & have trouble breathing, this what they call "demons". & no, this has nothing to do with actual demons or whatever.
I'm 26 & I have had these since as long as I can remember. I get them once or twice a week, & the "demons" occur every few months. I'm use to it. But if you're experiencing them frequently & not liking it, you can see a doctor. They'll do some sleep tests & whatnot.