My experience with going to bed is often my feet are already cold. Add to that I'm still building up the heat in the bag. When I use the piss bottle I can put it down with my feet and they warm up. As for keeping it all night and maintaining the heat? Probably could get a couple of the rocket scientists to make a better bet on that. But I know it works for me.
It also means I don't have to get out of bed in the night and lose what heat I have stored up.