Forget about handwarmers in the bottom of your sleeping bag. I learned this one at Mountain Warfare Training in the Marines. In the evenings we would get "hot wets" ie hot water or broth. After everyone had their fill, those of us in the know would fill our canteens with what was left and then put the canteens in the bottom of our sleeping bags. Just have to make some extra hot water for cocoa in the eveing.