Well, I can imagine that allowing your brain to think consciously while it's not necessarily supposed to can make the body put paralysis on overdrive, so you don't wake up accidentally, but that's just me taking an artistic license in biology.
I'm perfectly content to lucid half-dream, (staying awake, but toning down your thinking enough so that your subconscious kicks in and does all the weird shit a normal dream does while you're still awake and thinking. Not sure how else to explain it...)