text: why do we immediately start dreaming when we pass out? i've passed out twice in my life and each time i started dreaming. of course, i didn't realize i'd passed out until i came to but it was much more disorienting because i'd been dreaming and didn't know what was going on at first. i talked to a friend about it and he said that he started dreaming when he passed out once at the doctor's. so why is that? rem sleep is initatiated by the 'pons' of your brain.  it releases a chemical acetylcholine, which then causes the other body reactions to begin.