Ah... sleep paralysis. Used to suffer from this a lot. Strange thing is that once you are used to the terrifying state, it can become useable, in a way. Some people imagine death/aliens/a succubus are crouched on their chest, but once you know whats going on it can be an interesting (tho still unpleasant) pathway into a variety of dreamstates. The best way to describe the basic sleep paralysis is that you are locked in behind closed eyes, but are very much awake. You imagine you are in your bed, inside your room, represented as a fairly believably real space, but things are sitting on you, heroin demons kissing you, your immobile body being flung spasmodically around the room like a pathetic rag doll. Escaping this torture is hard, as even if you awake briefly you will feel yourself being sucked back in. Something unique about this state is a humming sound, and a buzzing, numbing feel in your body, almost like a massive drug high (or that's how I eventually rationalised it, certainly made it slightly more enjoyable)... as you slide into sleep paralysis the first sign is this humming sound, this buzzing feel inside your body, growing louder, stronger, and then you're in, and this buzzing is almost addictive in this state, extremely difficult to break away from. I presume that it is the sensation of having no feeling in yr physical body. This humming sensation is often widely reported in alien abduction cases. The upside of sleep paralysis is that it enabled me to enter various more interesting states (from lucid dreaming to what I call a zero-narrative dream state). The latter of these was the most shocking of all, a dream which rather than merely being a narrative invented as your mind flits out of dreaming and back to wakefulness, was a pure stream of randomised information, perceived in real time as virtual meta-schizophrenia, where every point of information synaesthesically cross references, and draws metaphors with every other.
@Bruno: From experience the best way to properly escape (rather than the multitude of half-escapes which characterise this state) is to keep food or drink by yr bed. If you can manage to eat or drink, for just a few seconds, it wakes up yr metabolism and seems to mean that you do not slide back in. The difficulty is, of course, to get even awake enough to do this.