"Every night, when we fall into dreamless sleep, consciousness fades. With it fades everyone's private universe - people and objects, colors and sounds, pleasures and pains, thoughts and feelings, even our own selves dissolve - until we awake, or until we dream." - from the Preface to Phi, A Voyage from the Brain to the Soul, by Giulio Tononi
I can confirm that such is my experience, too. When I fall asleep, there still seems to be a dim consciousness, as when I awake from this state, I'm aware that I was asleep and that time has passed. I have some memory of being asleep and an awareness that some period of time has passed, although my mind is a very unreliable indicator of the amount of time that has passed.
Several years ago, I underwent some corrective surgery for a deviated septum. It was the first time that I had been anesthetized, totally put under, since childhood. What is interesting to me is that as I look back I realize that while I was under, there was absolutely no consciousness or awareness of a passing of time - it felt as if I had been put under and then instantly awakened, although somewhat groggily, with no interval of time between, although I later came to realize that I was under and in surgery for well over an hour or more. There was no memory at all from that period, no recollection even of unconscious blackness. This is very different from an evening's sleep - when I put my head down on the pillow and close my eyes, it does not feel like the alarm clock instantly goes off.
So it seems that there are deeper and deeper levels of unconsciousness - the dim consciousness of dreaming sleep, the deeper unconsciousness of dreamless sleep, and the total unconsciousness of anesthesia. This brings up the question, then, are there inversely higher and higher levels of consciousness while awake?
It seems that we have moments of heightened awareness, be it the adrenalized state of fear or excitement when all of our senses are working on overload in response to a threat or perception of imminent danger and time seems to slow down, or moments of bliss or ecstasy when we seem to be transported out of time altogether and our mind is focused on a single sensory sensation, be it tactile, aural, or visual.
I wonder if these states of heightened consciousness are akin in an inverse kind of way to the successively deeper states of unconsciousness (sleep, dreamless sleep, and narcosis), why the perception of time seems to be so intimately tied to these various states, and what all that tells us about the nature of time, of consciousness, of memory, and of reality itself.