Here's a good recent article about this creepy phenomenon

http://www.cbsnews.com/news/the-real-life-nightmare-of-sleep-paralysis/

Notice the difference in the explanation between dreaming and hallucinating. There's a big difference. Those who say they're dreaming are having nightmares, that's not sleep paralysis. True sleep paralysis is when your brain has woken up before your body has had a chance to. I don't doubt the hallucinations are scary,in fact I experienced that once myself, but if you're just having nightmares consider yourself lucky because you have not experienced true sleep paralysis. And trust me you really don't want to.

There's nothing fun about feeling like you're paralyzed, even if it's just for a minute or two.

~What if this is as good as it gets?!~