For some, when you sleep your spirit flees from your body. In order to that the sleep paralysis would happen because of the spirit that didn't get back well to the body, leaving you between this and other reality. According to this theory, if you've ever seen any kind of spirit, it is because of that other reality.
To get off of sleep paralysis is not hard, begin blinking your eyes, getting more control of your face; moving fingers, then hands; toes, feet; soon your arms and legs and then your hole body, and you're off of sleep paralysis. Hope I helped, sometimes it feels horrible to not control yourself and see things you don't want. Some say you can actually make use of it to access that other reality, and becoming bond with it; but the choice is yours weather you want to keep with it or not.