"... when hypnogogic hallucinations are present, people feel that someone is in the room with them, some experience the feeling that someone or something is sitting on their chest and they feel impending death and suffocation."
This first happened to me when I was about 12 years old and and completely freaked me out. Over the years I researched these symptoms and figured out that I was experiencing was a documented syndrome called Sleep Paralysis. This happened to me continuously when I was growing up and can happen sporadically these days. I am able to control it better now and don't freak out as much as in the past. I just concentrate, control my breathing and the let out a big breath to help "stun" my body into waking up.
Sleep Paralysis | Stanford University