Real late to this and it will probably get buried, but sleep paralysis. For those that don't know what sleep paralysis is, it's when you half wake up and your brain still thinks it is sleeping. You can move your eyes around the room, but can't move your body, and you can have very vivid hallucinations.
From what I've read, your brain shuts down movement when you sleep to prevent your unconscious mind from acting out your dreams. Its a safety mechanism so you don't accidentally run off a cliff or something when you sleep. The problem is you can wake up and your brain is still in "dream mode". You can't move, and you can still "dream" and hallucinate things that aren't there.
Most of the time hallucinations are pretty mild, such as shadows moving around the room, or weird noises. But sometimes they are scary as shit...
My worst episode happened a couple years ago. I was dreaming about hanging out with my friends in a basement. I felt a hand on my shoulder and went to turn around but the figure moved with me to stay at my back. I panicked and whipped around to find a little girl. But it wasn't a normal little girl, it was something out of a horror movie. Think of something like the girl from the ring. More panic. My first reaction was to hit her, but when I tried it was like my body was frozen. Her eyes lit up and she let out one of the scariest blood-curdling screams I have ever heard. I instantly woke up.
But I couldn't move. Ok I've been here before, nothing new to me, breathe deep and it will pass or I will fall back asleep. And that's when I felt it. My covers were being slowly pulled off of me. I tried to sit up but it feels like you are buried under thousands of pounds of dirt. I strained my muscles as hard as I could and nothing.
The more I struggled the more I began to panic. Which is by far the worst thing you can do. Hallucinations began to get worse and I began to see dark figures sporadically move all around me. I then heard a deep booming voice, that sounded like some demonic voice you hear in horror movies. I couldn't understand it, but it was screaming at me. Its hard to describe but I could tell I wasn't "hearing" it. It was in my head. It's like that voice in your head as you read something, only it wasn't mine and I wasn't controlling it. After that I snapped out of the sleep paralysis and everything was fine. I was breathing really heavy from straining my muscles for so long, and sweating pretty bad. I didn't sleep for a couple hours after that.