Why do I always fall into deep sleeps?

I fall into these very deep sleeps and I'm only 18-years-old. This has been going on for about a year or two. What happens is that I fall into this deep sleep, I can hear everything going on around me, I can't move, and my heart is beating extremely fast; I feel like I'm in a coma. Anyway, I can't wake myself up and I have to talk myself into being calm again in my head. I scare myself so bad. What is wrong with me?