Kingruv

Why the issue?
Pt1
Physiology:

Factors: Cold weather reduces sweating because of vascular constriction. Since the extremities are thinner compared to the core they of course cool first. The body is concentrating blood in the core. Since there are more nerve endings in the extremities you'll also sense this more readily. This is a self preservation mechanism and warns you by discomfort. Heat loss through the head, neck, face and eye sockets, as well as through breathing, increase loss. These all need to be mitigated by covering.

In addition to this, if you have been expending energy before sleep any sweating you've done will cause further cooling. Waste build-up in the blood exceeds in cold weather that of hot weather. The skin is able to flush (literally) through sweat, during warm weather providing ventilation directly to the limbs and core to some degree. Again the lungs through breathing helps cool the core some. As the core tries to retain more heat, the liver and kidneys continue working, filtering blood. The brain, if called on for concentration, signals the kidneys to reduce blood plasma of some water content. This in turn goes from the kidneys to the bladder for eliminating.

Now, to the likely why. Since there is some muscular contraction during cold weather and a higher production of urine, you are likely to feel the urgency sooner rather than later. The muscles around the pelvis contract along with the rest, putting more tension on the bladder.

This is a guess, but I think the heat loss issue is a time problem. As the bladder fills, the urine is warm. This warms the bladder which in turn radiates some heat outwards lending heat to veins returning from the legs where blood has cooled. The gain turns to loss over time though. When lying down and inactive, the body is allowed to divert more energy to the issue, again raising urine production. Getting out of the bag certainly doesn't help.

Pt 2
Consumption habits
Ideally, water which is at least warm is the best thing to drink during cold weather. Blood always needs more water for replacement. The first, highest priority for this is brain fluid. In a healthy person the most purified water becomes brain fluid which in turn becomes spinal fluid and passed on to the nerves. Anything that mitigates this such as alcohol, aspartame (Nutrasweet), caffeine, and a few other items, diminish the benefit from water combined with the above.
You might be surprised I mention aspartame. When aspartame is consumed, it chemically breaksdown into methanol which of course is poison to the body. In a healthy person, and since it is a somewhat small amount by episode, the liver is able to deal with it by changing it into formaldehyde, another poison, but a convertible waste the body can eliminate. This takes energy you already need for heat production. (do you get headaches when using foods or drinks with Nutrasweet? It's from the methanol.)
The effects of alcohol on this should well known enough I won't get into that.

I think caffeine is a mixed bag. It is a stimulant and has diuretic effect. On the other hand products such as chocolate and coffee can be ok and maybe beneficial as long as someone doesn't have a low threshold for side-effects. Achillies is right about the candy bar. Just don't pick something too sweet. Chocolate covered raisins and cashews are good. Because of the high fat content, it makes a prolonged burn during the night. Kind of like banking a fire before bed.

Drinking to make sure you don't sleep too long can be used beneficially. The problem I've had was getting into the habit to make sure I had a early morning call about 4, that as I've gotten older and don't need to get up super early, I forget at times and by force of habit still drink too late sometimes.

There have been plenty of times in bygone days, I have gotten on my knees with my bag pulled up to my shoulders to allow just enough room to hold a pee bottle without getting out in the really cold weather.
As to staying warm, prepping the ground under your bag goes along ways to helping this. (bury a few dry, softball sized rocks heated by campfire below your feet about 6 to 8 deep. If there are needles around, pad the area under your sleep area with them.)

As regards to wearing in a bag, I'm NOT a tranny and I do wear silk longhandles still at times. It was the defacto superthin underwear for waterfowl hunting before polypro. It won't keep you warm as wool if wet but otherwise it's as good. It's also lighter and thinner to pack, wash and dry and other issues pertaining to travel outside the country.

From the FOG (frackin old guy)