They suggest winter campers keep their boots warm at night by sleeping with your boots or boot liners. You can use a Nalgene water bottle or drinking bladder to make a hot water bottle for your hands or feet. They last for hours. The list says winter campers should drink at least one liter before starting on the trail and then drink every half-hour thereafter. Have water to drink during the night because, when you are dehydrated, the body spends much more energy keeping warm.
Dehydration translates into a lowered blood volume or less warm blood circulating.
Your heart must work harder to pump blood to the extremities as a result. Another important tip is after consuming all these liquids, let it go. The experts say it takes energy to keep urine warm in the bladder. It's the same with the large intestines. They say plan on getting up at least once in the night.