Warming a tent is very, very inefficient. Not only is there too much air inside that needs to be warmed, but you'll lose heat to conductivity almost as fast as you distribute it against the thin nylon fabric of the roof and walls. Tents are far worse insulated than a snow cave!

The sensible thing to do is to insulate yourself, using clothes, hat, gloves, sleeping bag, and so on. The hot water bottle idea is an oldie and a goodie. The snuggling idea is even older and better tested, plus it meets with near-universal approval from those who try it. <img src="/forums/images/graemlins/laugh.gif" alt="" />