I suffer from occasional bouts of sleep paralysis, but thankfully they are very mild. Most of the time I can even “shake” myself out of a spell, by wiggling my foot until I build up enough momentum to wake myself. (Restless leg syndrome has one advantage, at least.) I’ve never experienced any of the hallucinations that many sufferers of sleep paralysis experience.
So, are they just products of the not-fully-awake mind, as would seem the most likely explanation? Or is it that, while in an altered state of consciousness, people can perceive things that are normally invisible to them—and those things can perceive the people in return? The most fascinating accounts of sleep paralysis, and the most troubling, are those where outside observers allege to have also witnessed the phenomena. One such case is briefly explored in the documentary THE NIGHTMARE, but all too briefly. While the film is fascinating, I would have liked to hear more from the scientific side as well. As it is, the documentary only offers recountings and reenactments of incidents of sleep paralysis. Those reenactments, though, are enough to satisfy Horror movie lovers, which is, I expect, the audience they were aiming for.
Sleep paralysis could go a long way towards explain how the vampire myth came into being. If one suffering from a bout of it hallucinated a vision of a recently-deceased relative—especially likely if that person had been conditioned to expect such a visitation—then you get a readymade vampire.
But what *about* those cases where someone *other* than the sleeping victim sees something?