In high school I and my friends built scenery for the school's shows. Fun with power tools. A couple of months before one show, the auditorium was temporarily condemned because it was full of asbestos. We were told that we would be able to get back into the auditorium only three days before the show was supposed to open. Installing a set, doing the wiring and the lighting and otherwise getting the theater ready for the show is a massive job, so we just decided we would stay up all night a couple of nights in a row and get it done. 5AM or so I fell asleep in one of the seats, then slept just an hour or two, only to be woken by a loud strange noise. I had no idea what it was. Crashing sounds and weird warped howling. I got up, looked around, couldn't figure out what the hell the noise was, but it came from the loudspeakers. Several seconds later, something in my brain 'switched on' and I instantly recognized the beat, the notes, the words. It was Sgt. Pepper's Lonely Hearts' Club Band. I knew the song well, but when I thought back on what I had heard moments earlier, I remembered it as atonal, arrhythmic, non-linguistic noise. My brain, unable to find the pattern, encoded it in memory as largely patternless, a snapshot grossly out of focus.
What this episode drove home to me is that saying that someone is half asleep, or only partly awake, is often literally true. Some parts of the brain may boot-up, come on-line, get in-gear, be hooked-in (etc.) more quickly than others. My ability to stand up and investigate the source of a sound may precede by several seconds my ability to process that sound. Or someone being read to at night may lose the ability to remember what is going on in a story long before she is asleep enough to not understand each sentence. Some bits of my brain don't seem to start working until after breakfast, and I don't even take caffeine. I've more than once had the experience of waking up and being totally unable to move for several seconds, with mobility then returning over the course of two or three seconds in different sets of muscles.
All this comes to mind because of an interesting article in New Scientist, on the non-monolithic nature of sleep. I recommend it, because it makes very clear how overly simplistic many models of human consciousness are, and how little we understand.