The word "most" is used unnecessarily.
It most definitely is...
Also known as a "Hag", "Shadow person", and other dark spiritual beings. But thats for the superstitious.
Well I don't know much about sleep paralysis but you said it happens when you wake up from a dream. So I'm pretty sure I already know what happens.
When you are going through REM sleep your brain paralyses the body, and with good reason. Scientist have deactivated the part of the brain that paralyses the body on many different animals including humans. In cats while they were dreaming they would get up and they appeared to be stalking prey. Dogs would appear to be running (though they stayed lying down) People would get up and start walking around, climbing on top of things ect. This paralysis is very important to our safety but I'd imagine if you wake up mid REM sleep and the paralysis hasn't shut off yet it would be quite scary. Especially if you didn't know the information I just told you.
A more interest thing though.... Why has our body gone to such great lengths to allow us to safely dream? What is so importanat about dreaming that our body's created a chemical to allow us to do it safely instead of just getting rid of it all together?
I've also heard that it happens because your spirit leaves your body as you dream and if you wake up before it returns, that happens. Though, there is a completely different scientific explanation as to why people have sleep paralysis.