I have suffered from sleep paralysis several times, and it’s a really scary thing that I’ve never knew how to explain, but this is a very good explanation:
“At first, you can’t move. It feels like you’re waking up, except your arms and legs and head and tongue are all frozen in place. You want to cry for help, but you can’t. You’re betrayed by your body, paralyzed. You lie there as your breathing begins to quicken, your heart rate jolting, until you see it: a shadowy figure in the room, moving closer. Maybe it’s a man in a dark cloak. Or maybe it’s an old woman, grotesque and witchlike. Either way, there is something sinister—and you are there, in your bed, powerless to do anything.
This real-life horror is known as sleep paralysis: a half-dreaming, half-wakeful state that leaves your body immobilized while you encounter nightmarish visions of terror. By some estimates, it affects about 6 percent of the general population, many of whom are left without an explanation for what happened to them. Was it a dream? Was it reality? Was it supernatural—and will it happen again?”
“Rodney Ascher’s New Doc ‘The Nightmare’ Reveals the Real-Life Horror of Sleep Paralysis”, Arielle Pardes, Vice.
I’ve had several experiences of this kind, mostly related to the type of sinister figures described in the article, but I remember specially one occasion in which the hallucination was auditory, not visual.