On most of the hunts I go on, especially backpack hunts, I carry a couple of Nalgene bottles. Not only are they tough, but they can serve as a sleeping bag heating catalyst. It's about as simple as it gets, but makes all the difference in the world. When the mercury is dropping hard at night, just boil a pot of water right before you go to bed. As soon as the water is boiling, dump it in an empty nalgene bottle, tighten it up, and throw it in the bottom of your sleeping bag. When you crawl in, your feet will be toasty, and will continue to stay warm throughout the night. Plus, the bottle acts as a fairly long-term warmer. The plastic provides enough insulation to keep the hot water from burning your feet, and also produces a slower, steady release of that heat. The area around your feet quickly heats up to the same temperature as the bottle, and once that thermal equilibrium is reached, the only heat lost from the water is the little bit that is lost through the insulation of the sleeping bag. I know we can all do without a lecture, but I have to use my engineering degree for something, right?