BrianLe,
I have been thinking about the influence of altitude lately on such things as sleeping bag efficiency/ratings and other factors, but somehow the SAME temperature at HIGHER altitude is "COLDER". I live at 4,000 feet and camping in my backyard the other night at 18 degrees, I never even zipped up my sleeping bag. I can remember winter storms in the Sierras with temperaturs in the teens around 8,000 feet and it was already bitterly cold and we wore down suits in camp similar to 8,000 meter suits and I definitely zipper and up and closed down my sleeping bag.
Jim