You wake up, but you can't move. And there's some strange presence near, maybe right on top of you... you want to scream, but you can't...

Having experienced this terrifying event about half a dozen times (that I can remember), I found this Metafilter thread fascinating. Specially Oneiros' comment, as it mentions another aspect of these dream disorders: the sudden rush of imagery as one falls asleep. The description reminded me of what 'jackin' in' might well be like:
"Imagine your whole perception being an insane slideshow of what I thought were memories. Random, disjointed, and alien to me. It was like looking through another's life if it had somehow being stored in still pictures and short movies, but they were too many, too loud and too fast. And emotions!"

Usually, dozing before falling asleep, I experience that perception of random imagery: it can be a puzzling sensation, unsettling at times.
The detailed recounts of some people, describing every subliminal terror from cultural folklore, past and present, are quite interesting. From some external links' possible explanations of the phenomena, from the psychological, clinical, spiritual (the soul being tethered by a silver chord... soul delay, anyone?), the experiments producing similar experiences, initiated by magnetic fields, strike me personally as one likely path to explain it all. Energy is all around us; it may leak from and into our own brains with us not being aware of it; some people may be more sensitive to this.
Plus, look how we've flooded the spectrum with all kinds of information on the aether... who knows whay we're doing to our brains in the process...
/puts on tinfoil hat & pajamas, triple checks bedroom door

I've had sleep paralysis before, strangely only when I doze off sitting at a desk especially in my college's library . I was never really scared by it, more embarassed really, cause the presence that I would feel near me would be my friends making fun of me for sleeping in the library, or the librarian something.

One time it was some strange man who's skin was black, in the sense that it seemed to reflect no light. I was unable to move on the desk, and he just spoke some garbled words and then rubbed my nose.

Carl Sagan's The Dragons of Eden has much to say about sleep paralysis and hypnogogia; essentially, he claims that these phenomena can account for the vast majority of alien abduction experiences (as well as many a mystic vision, etc.)

Interestingly, just like one can train oneself to dream lucidly, it seems one can train oneself to extend and direct hypnogogic states as well.

Sleep paralysis occurs in a percentage of "normal" people. It is more commonly found with narcolepsy and some other conditions; with narcolepsy it is also accompanied by decreased REM latency (dreaming as soon as you are asleep), hypnogogic and pompnogogic phenomena (quasi dreamlike states upon falling asleep and waking up), and sudden loss of muscle tone (cataplexy) often in association with strongly emotional events.

Mystic visions are often ascribed to seizure activity in the temporal lobes, e.g. temporal lobe epilepsy aka complex partial seizures. These types of seizure are associated with the feeling of a religious presence, often times a lack of sexual interest, writing a lot, outbursts of rage, and olfactory hallucinations, along with some other symptoms. Joan of Arc, for instance, was thought to have such seizures. Transcranial Magnetic Stimulation, in which an MRI like magnet is used to induce an electrical current in the brain, can likewise induce the feeling of a spiritual presence when temporal lobe activity is stimulated.

To me it seems like there are all sorts of "hypnagogic" states, which don't have much in common.

A) The "rehash" - this occurs whenever, but ONLY whenever, I see unfamiliar visual stimuli during the day. For instance, I went for years without playing cards, then had a night of cardplay, and upon closing my eyes in the dark (whether or not trying to go to sleep) imagined hands of cards of all descriptions laid out and passing back and forth. Another time I went hiking around in Acadia National Park, and upon closing my eyes to sleep (after a five-hour car trip back from Maine) I suddenly had the vision of the distinctive rock formation I'd been hiking over as seen from the air and saw it as if I were flying around it from all sides - really very beautiful.

B) simple pulsations - any time I close my eyes and relax in a sleepy way I see rythmic pulsations of blue and yellow that mostly die out after a time. As I've gotten older the pattern tends to pick up more and more "noise", and it's my suspicion that it's associated with the retina or optic nerve at a low level.

C) true hallucinations - these I've seen only rarely, and usually on waking up rather than going to sleep. Mostly an almost instantaneous perception of some person or object, typically hanging in midair, that vanishes immediately because it sets off a fight-or-flight type response that banishes such foolery from mind.

D) simple dreams - this is perhaps a variant on the "rehash", but much more dimly illuminated and without obvious link to the day's subject matter. Just faint images of things shuffling around in the gloominess when eyes or closed or it is pitch (darkroom) black, which gradually intensify if concentrated on, or weaken if pointedly ignored. These seem like a very weak version of proper (sleeping) dreams, but they make use of the same "light" provided by pulsations (B). They also can be "powered by" sunlight if I close my eyes while the sun hits them.

Where sleep paralysis is concerned, it affected me repeatedly during 1-2 years, dozens of times, but never since. I found that it could be broken either by a very strong effort of will to move, or by triggering a sort of "shivering" response on purpose, sort of like what you'd do when cold. The former technique does not break the paralysis directly - rather, it's the proprioception of moving, I think, that would break the paralysis. So the first method wouldn't work if there was so much as a sheet holding my hand from moving, because even with a strong effort of will the actual force produced was miniscule. But the second method is easier anyway.

To me the most mysterious phenomenon with sleep is the sensation that comes at the exact moment when sleep "hits", after I'm already groggy and have already gone far down the way to dreamland. It seems like there's a particular moment when I go from merely lying very peacefully and detachedly to deep sleep, which involves a strong sense of tipping at a sharp angle, pressure in my head, and a rushing sound. My best guess is that something interesting goes on with the CSF at this moment... whatever it is, it's extremely refreshing - if I abort the sleeping at this time, I tend to wake up feeling as if I've had a fairly long sleep anyway. I think this same phenomenon might occur those times when I'm nodding off at a seminar ... several times I can nod off instantaneously and be awakened when my head starts to turn, but still find myself incredibly sleepy, but then one time I'll nod off and awaken, a mere instant later, feeling refreshed and without any sleepiness at all. I think this deep sleep entry sensation might have something to do with what makes that last nod "special".

Funny--I used to get sleep paralysis a *lot,* almost never now. It's the reason I stopped taking naps, because that's when I'd always get it. I suspect it had something to do with stress and not getting a normal amount of sleep--it nearly always occured when I fell asleep away from bed and during the day.

Trying really hard to move or shiver never worked for me--I distinctly remember waking up paralyzed near the edge of a couch and trying my damndest to move jst enough to roll off, hoping the shock of hitting the floor would wake me. I'd roll off...

and "wake" again, realizing that the rolling had just been a perversely limited sort of lucid dream. Best solution I ever managed was to justtry and go back to sleep, trusting that the paralysis would be gone by the time I woke again.