One thing to keep in mind - should anyone find themselves sleeping in a very cold situation - is to eat significantly more than normal, particularly fatty foods (butter!). In the cold, your body uses a lot of calories to keep your body temp up and eating a dense meal right before bed goes a long way to your overall comfort.

I also have a belief that keeping a hot beverage ready-made in a thermos (tea or just hot water) should be considered a safety item when winter camping, sea kayaking, etc. Warming yourself from the inside is more effective than trying to throw on extra layers.