I have heard conflicting theories about the best way to keep warm in a sleeping bag in the winter. Some people say you should bundle up in your sleeping bag and another theory says that you should be naked, because the warmth of your body will create a microenvironment of very warm air right next to your skin, since the sleeping bag and pad reflect heat very efficiently, and there is no way you will get that warm if clothes block contact with the air. Does this "naked" theory make any sense according to Physics?
short answer: No, the more insulation between a hot body and a cold body the slower the movement of heat from hot to cold.
long answer: If the clothes themselves are cold then bringing them into the sleeping bag requires them to be heated as well as the air/skin (which is what produces the warm feeling), so short term, go naked, long term keep dressed.
Let's start by assuming the sleeping bag is perfectly reflective and impermeable to heat, and that your body remains at a constant temperature (which it generally will unless you're very ill or in danger of dying from being too hot or too cold). In this situation, thermodynamics tells us that everything inside the sleeping bag will eventually come into thermal equilibrium with your body, reaching the same temperature as it. When this happens you will feel nice and warm (if not too hot) and will not be losing any heat to the outside world.
However, the time taken for this to happen is very dependent on what's inside the sleeping bag with you. If your clothes start out cold and have a high heat capacity then they will have to absorb a lot of heat from your body before they reach its temperature, which will take time. So if your sleeping bag is a perfect insulator then you should definitely be naked, because that way you'll feel the benefit of the sleeping bag more quickly.
However, even a very good sleeping bag isn't a perfect reflector of heat and will let a little bit of it through, especially if it's very cold outside. In this case the temperature will still reach a steady state, but it won't be a thermal equilibrium. Instead the system will reach a state where your body is constantly emitting heat, at a rate that's balanced by the rate at which your sleeping bag leaks heat to the outside world. The slower the leaking of heat, the higher the temperature next to your body will be. Your clothes are insulators and slow down the leakage of heat, so in this case keeping them on is a good idea, even though it will take a while for you to feel the benefit.
So I guess the tl;dr version is, you should be naked if your sleeping bag is very good and/or it's not too cold outside, and/or you won't be there for very long.
The relevant issue is that the clothes you wear (unlike the down or synthetic fibers of the bag) hold moisture next to your body. Changing both the thermal conductivity of the medium surrounding your skin allows energy transport to the environment. The evaporative heat loss dynamics of the body-bag system is described by another thermal conductivity.
I propose the effects of creating a moist fabric layer allows a more efficient heat flow from the body than an air insulator. Water transport through your fabric should be best in your sleeping bag, naked a should feel colder and you will freeze in wet cotton cloths.
protected by Qmechanic♦ Mar 1 '13 at 20:01
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