Hot Water Heat
Warm rocks in the sleeping bag work well to keep you toasty (use caution--don’t overheat). But sometimes rocks are hard to come by in a wilderness buried under two or more feet of snow. It’s not uncommon in the winter north woods to step off your snowshoes and sink to your waste or deeper, so warm rocks might just be considered a pretty dumb idea at the time. But water also holds a fair amount of heat, and can be brought into your sleeping bag in a number of ways. An old fashioned water bottle works very well, as does a wine skin. I prefer to fill my water bottle with hot water, since I like to have it in my bag anyway for that middle of the night dry mouth. In any case, the important thing is that they don’t leak. This is critical, so be darn sure, or you could spend the night wondering what kind of idiot would leave a perfectly warm house to sleep in a snow bank a million miles from nowhere.