You know when you are in your tent or bag and it's cold out and you have to go pee, but it's COLD out? Well go pee. When you keep extra fluids in your body your body is spending energy trying to keep that liquid heated to body temperature and from what I understand the body prioritizes keeping the core heated over the extremities. Not only that but going pee will let you feel relief and get back to some well earned sleeping.
* you can pee in a container with a tight seal (recommend bottles over ziplock bags) and jettison the contents and container in the appropriate places the next morning. I prefer to get out of my tent with my headlamp and check out the night and once in awhile get blessed with a shooting star.
I searched and searched but didn't see a thread on this topic specifically so I guess I get the dubious honor.