I will throw my 2 cents worth in. As a scout master I tought winter wilderness survival for 30 years. I have slept in many igloos on My Rainier and Mt St. Helens. I have also slept in many three season tents in the snow on the mountains. To stay warm at night: 1. Insulate from the ground (no air matresses, Thermarest good), 2. Get a good mummy synthetic (hallowfill, etc). If there is any chance of getting wet then NO down. 3. Sleep with a stocking cap and in your underwear (not fully dressed). Let the sleeping bag work the way it is supposed. If you have a bad sleeping bag then sure sleep in every thing you have. There will be a themal point near 32 deg between your body heat (hopefully 98.6 deg) and the cold out side. The key to being warm is to get the 32 deg point to be outside your bag. If you have a good bag and sleep in warm clothes, the 32 degree point (or "cold" point) may end up inside your bag. The real key is to manange where the cold transision point is. When I lived in Chicago and took the scouts camping at -14 deg and they had BAD sleeping bags, we filled the tents full of straw. (over them and under them). Just be sure no one has hay fever.
Ron Athay