It seems that people are worried more about insulation than the "furnace". The energy used to climb high can not be used to fuel the furnace. If you spend a lot of time in camp then the time between eating your evening meal and using your sleeping insulation is longer - this can NOT be good. Don't forget to keep a Snickers bar in your sleeping bag. A hot beverage warms you from the inside.
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"In theory, theory and practice are the same. In practice, they are not."
Yogi Berra