I have experienced this in the past. It is very scary. I have had the full "incubus phenomenon" with the feeling of a "presence" in the room, being pushed into the bed or crushed, and sometimes a weird sense of sexual arousal. I've even seen things coming "at" me, heard my name called, heard "things" coming down the hall along with clicking, banging or hissing noises and it used to scare the living crap out of me.
I'm sensitive to paranormal phenomenon, but even as a child I KNEW this sleep paralysis (even though I didn't know what it was called) was NOT paranormal.
My neurologist says it's more common in people with migraines, epilepsy and other neurological issues. But that anyone
can experience it when they are stressed, not sleeping well or ill.
My children have all experienced it, my middle daughter (who also has migraines, as all my kids do) has it the worst. She's learned in recent years how to shake herself out of it.
I was put on a medication for anxiety control and migraine prevention called Klonopin. Since I've been on the Klonopin, it waned and has virtually stopped all together. My neuro said he was starting to prescribe Klonopin for severe, unrelenting sleep paralysis, based on my experiences (kind of using me as a study subject) and has had good results with it. He said some of his patients experience it every night or every morning. (Mine were more common in the morning, when I would wake up and then fall back to sleep.) He says it's, in easy to understand terms, when "half the brain is asleep (causing hallucinations, as dreams) and half the brain is awake (being able to open your eyes and look around." The dream state is superimposed on reality, leading to a terrifying phenomenon.
I only take a quarter of a milligram of Klonopin a night and it controls some of my constant anxiety and almost all of the sleep paralysis.
I feel strongly that most people who think they have been "adbucted by aliens" or only have "haunted house" phenomenon when they are alseep are most likely experiencing sleep paralysis.
Look at alien type "abduction" stories; sleeping, hearing something approach you, down the hall or from the window or door, weird noises (popping, hissing, crackling, banging) a "presence" entering the room, then being assaulted. All of these correspond with reports of "alien abduction." My neuro was impressed with my deduction and said he was interested in investigating more reports of this phenomenon. (He's a sleep specialist (also a migraine and chronic pain specialist) and has the laboratory to do the research for this phenomenon.
Anyway, this is an interesting thing, but really nothing to be afraid of. It isn't paranormal, but can scare you terribly. There is medical care if you experience SP more than occasionally. Some people are afraid to sleep because this might happen. But there is a way to treat it.