Anacapabandit,
Here's what you do to stay warm at night while sleeping:
While you're huddled and shiveringly making dinner - and after you've put some hot fluids in yourselves, boil up a nice pot of water on your stove, and fill a spare nalgene with it. Make sure you screw the top on TIGHT. Don't want any leaks.
Dry the outside off, and put that nalgene right inside the middle of your sleeping bag. I fold the top of the bag down to keep the heat in.
When you get inside later, you've got a nice warm, cozy bed. It will keep you warm all night. I slept in a snow shelter this way in temps around 20 and it kept me warm all night. (Your mileage may vary)
Just be careful and DON'T BURN YOURSELVES! Might want to wrap it in some article of clothing at first. Oh, and I wouldn't suggest anyone try this with a bladder.