No. It’s a myth that sleeping naked in a sleeping bag is warmer than wearing in long underwear. I’m not sure how this Internet meme started but it’s dead wrong.
How Sleeping Bags Work
Sleeping bags are designed to trap the heat your body produces and prevent it from escaping. When you wear long underwear in a sleeping bag, you increase the amount of insulation between your skin and the cold air outside. This will make you warmer than if you sleep naked.
If that doesn’t make sense consider the following analogy:
A sleeping bag is like a house. A sleeping bag has insulation like a house that prevents heat from escaping in cold weather. Like your house, a sleeping bag has a furnace inside it that heats it up. In this case, its your metabolism which produces body heat. If you wear long underwear inside your house in winter, you will feel warmer than if you walk around the house naked. That is why wearing long underwear in a sleeping bag will keep you warmer than sleeping naked. It’s another layer of insulation that traps hot air and keeps it close to your body.
Exceptions to the Rule
There are times when wearing clothing in your sleeping bag will not keep you warmer. These are extreme exceptions, but I will list them here for completeness.
- You wear so much extra clothing or fill your sleeping bag with so much extra stuff that you compress the insulation in the sleeping bag’s baffles and reduce the amount of warm air it can trap.
- You wear such tight-fitting long underwear or socks that it reduces the blood circulation to your extremities and makes them feel colder.
- You wear wet clothing which compromises the insulation in your sleeping bag as the heat of your body dries it. The moisture in your clothes doesn’t just disappear: it gets trapped by the sleeping bag’s insulation which degrades its effectiveness.
- You wear too many clothes in your sleeping bag and sweat. As your sweat dries it degrades the insulation in your sleeping bag, just like wearing wet clothing. You’d have to sweat a lot for this to happen, so just de-layer or unzip your sleeping bag if you feel a sweat coming on.
The best practice is to wear a dry base layer (top, bottom, socks, and hat) in your sleeping bag at night to keep it clean and to keep you warmer in cooler weather. These should be loose-fitting to prevent your hands or feet from getting cold due to loss of circulation and to help trap warmer air near the surface of your skin.
While you can augment the insulation in your sleeping bag with an insulated coat or pants, you want to make sure that you can still move inside your bag and that the shell of the sleeping bag isn’t pushing hot air out of its own baffles. If your base layer is wet or damp, it’s best to dry it out before you get into your sleeping bag.
If you start to sweat at night, unzip your sleeping bag to cool off and re-zip it when you start to get cold. The amount of heat your body produces during the night changes, to a large extent based on how recently you ate food. So if you wake up cold at night, eat something sweet and fatty like a candy bar (without caffeine) or some nuts.
You have to understand that sleeping warm at night isn’t something that just happens. Your experience is very much influenced by the steps you take to sleep comfortably like wearing loose dry clothes to sleep, venting your bag when you’re too warm, or revving up your metabolism by eating something when you get chilled. Sleeping warm is a skill.
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