I've had my body fall asleep while my mind was awake before, but not the opposite. It's a very odd sensation feeling your body snore of it's own accord, with your breathing rate slowing down, hearing sounds in the environment. This happened a couple of times while I was trying to learn how to move from the waking state into the dreaming state (WILD technique in lucid-dreaming parlance). I managed it once, but it was a lot of work for me (lots of others can do it much easier than I can).
