> Peeing in the middle of the night might suck but you will be warmer in the long run.
> Your body will not burn calories keeping unneeded fluids warm. It really does make
> a difference.
> It really does NOT make any difference at all. Sorry, but that is just a myth.
Roger, you're spot on thermodynamically (as usual). However, I've heard of a physiological reason why holding fluid in your bladder may in-fact make you sleep colder. The reason is that with a full bladder, your body will push more blood to the extremities, defeating the body's mechanism of preferentially cooling the arms and legs to preserve "core" warmth. Ironically, if this is true, having a full bladder may make you feel warmer as your hands and feet will have more blood flow, but your total heat loss will increase and you'll ultimately end up colder. :(
Sorry, I can't cite a reference at the moment, so we may have to chalk this one up to folklore as well. ;)
Oh...to the OP:
If you sleep in a floorless shelter, you can just roll over and pee in the snow. Dig a little pit if you're self-conscious about it.