Everyone experiences sleep paralysis every time they sleep. It is a normal function of the human sleep cycle and is a built in safety against physically acting out your dreams while sleeping.

What the OP is talking about though is Awareness During Sleep Paralysis or ASP. This occurs when the sleeper moves into a semi-awake state before the paralysis wears off. It can be a very frightening experience for someone who is unaware of this paralysis function (which unfortunately is most people).

My first experience with ASP was one of the most terrifying experiences of my life. Mine was the black shadow man/men.

I set about learning as much as I could about ASP and experienced it few more times with great fear. Eventually I learned to control my fear and the black shadow ceased to appear during ASP. Instead, I am now able to utilize the ASP state to embark on OBEs and Lucid Dreaming.

I strongly believe that the ASP aliens, hags, and shadows are nothing more than the conscious minds projection of fear of the unknown. Once you face that fear and overcome it, the door is opened to incredible realms of self exploration and understanding.