I think it’s one of those phenomena that’s common enough for a significant number of people to experience, but still rare enough for few people to know about it. The lack of intersection between the two groups is what gives rise to folklore.

I’ve had it a few times, but not regularly. The one I remember best was absolutely classic Incubus: I was seemingly conscious, aware of my surroundings, but paralysed by a crushing, invisible weight on my chest. And I was convinced, while in that state, that I was being attacked by Yog-Sothoth, which only goes to prove that it’s your own mental landscape that provides the ‘cause’: in times gone by, evil spirits, and aliens these days.

Once I’d awoken, and was lying there trying to remember how to breathe, I remember the small area of my brain that was still rational connecting the experience and what I’d read about the Incubus legends and so-on, and thinking: “Well. That explains a lot…”