There doesn't seem to be a sharp line between sleep and wakefulness. I once fell asleep while driving and was awakened by something I saw. It's an odd sensation to be woken up by something you see instead of by something you hear or feel. I had fallen asleep with my eyes open but was staying upright in the driver's seat, maintaining my speed and staying on the road. It was an extremely frightening experience, because I was very keenly aware of not being conscious a few seconds earlier, and having no sense of how much time had passed. I couldn't even remember what I saw, because duh, I was unconscious when I saw it. It freaked me out so much that even fifteen years later I think about it every time I get the slightest bit sleepy behind the wheel.

A few times in extreme states of sleep-deprivation, I've been lying down or slouched on the couch and realized, after being startled into getting up suddenly, that my memory of what I had been thinking about was fading away just like a dream. My eyes were open the whole time, and unlike when I was driving, I was conscious of seeing and having a consciously driven train of thought. Yet in some way I was asleep, so my mind let my memories slip away as if they were dreams.