I get awesome hypnogogic hallucinations when I stay up too late -- usually this happens when the person in bed with me is trying to continue a conversation, or trying to encourage other bed-centered activities, when half my brain has already fallen asleep. So I usually announce them ("Hey, I just saw a bunch of Vikings sitting around a big oak table eating porridge!"), and then Andy asks whether I'd rather fall asleep all the way or try to wake back up for a while longer.

(Yes, that is an actual example.)