If you were in a true survival situation, like a grossly inadequate sleeping bag, or out of food and conserving energy as much as possible, I think the thing to do would be to carefully pee into your nalgene bottle or whatever and keep it in your sleeping bag with you. Better than wetting yourself, or getting a poor sleep. If not surviving, I think it is better to get up and do it outside, no matter how cold or wet it is. It's good for you, and it gives you a chance to see what it is like outside. Don't step off the side of a mountain or something like that though. It happens. Know where you're going to go. Nalgene bottle or whatever can be a good routine if the situation calls for it. Keeping it in is pointless, regardless of the thermodynamics. I have a pretty good capacity, but when nature calls once, you can shrug it off, but when nature calls again, its time to get up and dance.