In four years of Arctic glacier research, I've spent over 150 nights camping on snow, in temperatures below -20oC. That might sound crazy, but I slept soundly in a cocoon of warm safety.
How does a sleeping bag keep you warm?
Like a lot of things, a sleeping bag is an engineering solution to a science problem. Let's describe the problem by examining the facts:
- The human body needs to maintain a specific temperature range: 37oC ± 0.8oC
- The human body produces thermal energy by unlocking the chemical energy stored in food — a metabolic process called cellular respiration.
- During sleep, the body's metabolic rate decreases.
- Heat flow transfers thermal energy from regions of high to low temperature.
In cold environments, the air around you is at a lower temperature than your body. Therefore your body heats the air around you, losing thermal energy in the process. This might be OK when you are awake and active (generating more thermal energy), but when your body is at rest, it needs a little help minimizing this energy loss.
A sleeping bag meets the requirements: thermal insulation, designed for the shape of the human body, which you can compress and easily carry to where you need it.
Did you know? Insulation is a material that resists heat flow.
The key to insulation is to form a layer of dry, unmoving air around your body. If the air moves, it transfers thermal energy away. This is why modern sleeping bags look big and puffy — they are basically a nylon bag full of a loft material that helps form pockets of trapped air.
Natural down fibers (the under-feathers of ducks and geese) provide high loft and are highly compressible and lightweight.
Synthetic fibers (man-made to mimic down) while slightly less lofty and transportable, have a single advantage: they maintain some loft (and insulation value) when damp, while down does not.
While you never plan to get a sleeping bag wet, it's something to consider when selecting a material.
Elert, G. (2005). "Temperature of a Healthy Human (Body Temperature)." The Physics Factbook Retrieved 01-11-2009, from http://hypertextbook.com/facts/LenaWong.shtml.
White, D. P., J. V. Weil and C. W. Zwillich (1985). "Metabolic rate and breathing during sleep." J Appl Physiol 59(2): 384-391.
"Heat Transfer Mechanisms" Retrieved 01-11-2009, from http://en.wikipedia.org/wiki/Heat#Heat_transfer_mechanisms
Article first published on December 9, 2009.
Photo credit: Wikimedia Commons