If they're going to hold off, I'd like a set time, and everyone knows if they want a pinch hit, to be there then. It's nothing to stay up once or twice. Even twice a day, and if that time goes with no emails, then that's it until the next time. Should still get most everyone, but lets people manage their schedules more effectively.

(says me, who's been awake for 30-something hours and refuses to sleep in case of pinch hits)