Regarding the "feeling", I'm personally of the "scientific explanation for everything" camp.
Sometimes it's fun looking for the explanation though. The "Hag" was a very common folk legend where I grew up. In Mexico it is "Subirse el Muerto" (Dead Person on You). In Japan it is the Kanashibari (Bound in Metal). In China it is the Guǐ Yā Shēn (Ghost Pressing on the Body). The Hmong call it the Dab Tsog (Crushing Demon). In Sri Lanka, the "Amuku Be" (Ghost that Forces One Down). The list goes on and on. Almost every culture identifies this phenomenon.
The 1781 painting "The Nightmare" is thought to be a classical depiction of such an occurence:
Local legend in my heavily superstitious southern upbringing was that it was an evil spirit that visited you at night. You'd awake with a crushing pressure on your chest, and you'd see and/or hear things. Sometimes you'd see an old hag (hence the name), an imp, succubus, or other critter sitting on your chest (likened to the crushing feeling). Usually it was said to be an omen of an impending tragedy about to happen.
When people would tell the stories, some people would believe them, some would say they were full of it. When I was a teenager, it started happening to me. I'd wake up, unable to move, and see chains holding me down to the bed. Woke up once on the couch and heard steps walking around in the kitchen behind me. Woke up once and couldn't move, and heard downright EVIL growling and cackling down the hall. Another time I woke up and felt a sense of dread - again, unable to move - and then felt a sharp claw plunge into my stomach. Another time at college, I slept on a loft bed (like a bunk bed but with no lower bunk - just a desk under there). I saw something evil looking (kinda like a zombie, or like the girl off of "The Exorcist") at me from the foot of the bed - just from the nose up. It keep jumping up and down as if it wasn't quite tall enough to see me up on the bed without jumping.
Scary stuff, PARTICULARLY when it was happening to me. That said, I started researching the topic. Turns out - there's nothing supernatural about it. It's a medical phenomenon
. You see, when you're asleep, your body paralyzes itself - it stops signals from the brain from being translated to the limbs as a safety mechanism. Otherwise, when you're running from an imaginary lion at night, you'd be flailing about and kicking in bed
. When that paralysis fails, you get sleep walkers. Now sometimes, you get the opposite - rather than the paralysis not being applied when it shouldn't (sleep walking), sometimes upon waking up, or just before falling asleep, the body will enter a semi-sleep state. The paralysis kicks in - which is why you can't move. Your breathing is also not under your own control. Your body will breath on "auto-pilot" just as you would while you're sleeping, but you can't take breaths when you explicitly want to, hence the heavy feeling you your chest. The dream center of the brain also kicks in - but your conscious and awake. The result is that you're paralyzed, and the dream center of your brain starts superimposing visual, auditory, and/or tactile hallucinations upon the real world. Now, since you're unable to move, a sense of dread kicks in. You feel afraid and vulnerable, and so those hallucinations take on a threatening form. As far as you can tell the stuff is as real as can be (because afterall - reality is in the mind). It reality though, it's all just a chemical interaction. You may be seeing zombies, or hearing growls, but they're not really there.
Once I learned that, the episodes started becoming MUCH less frightening, to the point where now when it happens I no longer see scary things. I just realize "Oh, it's the sleep paralysis again." and try to start to break it (usually by trying to rock back and forth - sometimes it can take a minute or two to break out of though - one of the primary fears of people who it first starts happening to is that they've awaken permanently paralyzed).
I believe that most of our supernatural experiences can be explained with an in depth analysis. In particular, the "sense of dread" has been studied before. I wish I could find the link, but I have seen research before that established a link between such feelings and certain electrical and sound frequencies. They did studies in which they placed a fan or a other electrical device within a location set to emit that frequency. This was something that was inaudible to the ears. Despite it just being an electrical device, people in the test group (subjected to the frequency) were much more likely to report feelings of being watched, of presences, and of just a general feeling of uneasiness, than the control group which was not subjected to the frequency.
