I've had it once or twice. Basically your body always paralyses itself in deep sleep for rest / safety but it wears off in lighter sleep cycles before you wake or, if woken from deep sleep, there's supposed to be a signal that snaps you out of it. If that signal doesnt go hough properly then your brain wakes up before your body = sleep paralysis. Happens mostly to people who have odd sleep patterns. Not so much lack of sleep as lack of routine - e.g. It's very common for people who work a combination of day shifts and nights shifts so are constantly shifting their sleeping hours.
Weirdest sleep thing that has happened to me personally though... Wasn't paralysis. I was just drifting off to sleep one night when there was this sudden, insanely loud, "BZZZZZZTTTTTTT" noise like electricity that shot from one ear to the other as though it had shot across the room. I jumped straight up in bed with my heart absolutely pounding. It was some sort of ****ed up auditory hallucination
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