This doesn't happen very often, but I find it interesting when it does. Maybe someone can shed some light on what exactly is happening.

This has only happened a few times.

I start to fall asleep and feel my body getting "sucked" away into the dream. I can still hear and see everything around, but I can't escape the dream! I try to wake up by shaking my body or talking, but nothing happens. Then I felt my girlfriend shaking me for several seconds then I finally wake up.

She told me I was shaking, my heart was racing, and I was breathing differently. Basically the same things I felt and "shaking" was the only thing I could do to get her attention to wake me up.

After this transition I usually just end up in a full fledged dream if I'm not awoken.

It's like I'm fully aware of what's happening as my body is trying to fall asleep and still can manipulate it.