There's this strange feeling I experience every now and then when I'm...asleep??? Well let me explain....

This happens after I've been asleep for some time. All of a sudden I can hear (I mean in reality, now) and can sense everything in the real world. The biggest problem is that I have NO power to move any part of my body at all, I can't even open my eyes. So basically i'm laying there still with my eyes closed kind of trapped. It's a very scary feeling and I was just wondering if anyone has ever felt this or knows what it is. I kick myself out of it by mustering up all possible strength (well it seems like that) and kind of jolting myself any way that will get my body to move. soooooo strange. help??