A combination of three:
-biological: I'm starting to feel tired, so I stop.
-logical: I suddenly realize that I haven't stopped playing because of the biological reason, so I just say to myself: "ok, I make sure that this wonder is mine, and I leave"
-psychological: two wonders, five techs and a war later, I feel guilty for having no will, and quit playing. A quick glance at the clock helps too.(the one I have voted for)
I tend to avoid the social reason by playing only after everybody is asleep. Not really healthy.