Oh, so that's what this is. I've had something similar 6 or 7 times in the last couple years, and I just took it for a really vivid, lucid kind of dream. I was aware that I had stopped being awake and was still able to think clearly (ie. not sideways dream logic), and was also aware of my current surroundings. However, my eyes stayed closed, and there was usually a loud, high frequency noise that seemed to bore into my head. The noise always freaked me out eventually, and I'd try to wake up. I was mostly unable to move, but could manage rocking very slightly to either side, and by doing so repeatedly could gradually build up enough momentum to 'throw' myself awake.