I’ve always heard that the best way to stay warm in a sleeping bag is to zip it up and sleep naked. I’ve been told by both military personnel and civilians that sleeping without clothes in a sleeping bag is warmer than wearing long underwear. I even overheard a surplus store employee advising a customer to sleep naked in their bag. However, based on my own experience, I’ve found that this “sleep naked” advice is dead wrong.
How Sleeping Bags Work
Sleeping bags are designed to trap and prevent the escape of air that is warmed by your body heat. The goal is to create a barrier or layer between you and the outside air or ground. Adding more clothing creates additional layers of air around your body, which helps to keep you warmer. The more layers of warm air you trap around yourself, the warmer you'll be. Wearing long underwear inside your sleeping bag increases the amount of insulating layers between your skin and the cold air outside, making you much warmer than if you sleep without clothing.
Is it ALWAYS true that more clothing will keep you warmer?
It is not always true that wearing more clothing inside your sleeping bag will keep you warmer. In fact, sometimes wearing clothing in your sleeping bag can make you colder, which could be dangerous or even deadly.
Keep in mind that air is trapped in the gaps or spaces within the sleeping bag’s insulation. If you wear too much clothing or stuff extra gear inside the bag, you may compress the insulating layer. This significantly reduces the amount of warm air that can be trapped, diminishing the bag's effectiveness.
Be cautious with tight-fitting long underwear and socks, as they can reduce blood circulation to your extremities. Reduced circulation will make you feel colder, not warmer.
Wet clothing also compromises the insulation of your sleeping bag as your body heat works to dry it. The moisture in your clothes gets trapped in the bag’s insulation, which significantly degrades its ability to retain heat. Wet doesn’t always mean soaking wet from falling in a river—it can also refer to small amounts of sweat, which can greatly reduce the sleeping bag’s insulating effectiveness.
The best practice is to wear clean and dry long underwear and socks inside your sleeping bag to maximize warmth and comfort.
Speaking of sweat
Wearing too many clothes in your sleeping bag can cause you to sweat. As sweat dries, it significantly degrades the insulation of your sleeping bag, much like wearing wet clothing. To manage this, simply remove a layer or unzip your sleeping bag to reduce heat buildup and prevent sweating.
What I wear in my sleeping bag
I use one of two sleeping bags: the Military Modular Sleep System (MMSS) and, more often, the USMC Three-Season Sleeping System (with cold-weather bag).
To get the most out of my sleeping system, I wear a dry base layer (top, bottom, socks, and sometimes a beanie or watch cap).
The clothing not only provides additional insulation but also keeps the inside of my sleeping bag clean, which, in turn, helps me stay warmer at night. On occasion, I have slept in my coat and insulated pants inside my sleeping bag and managed to stay well within my personal comfort zone. At times, I also use my woobie blanket (military poncho liner) as an additional layer of insulation. The woobie is one of my personal favorite pieces of sleeping gear.
Changing body temperatures
Sweating inside your sleeping bag is unpleasant and reduces its insulating effectiveness. Remember to remove a layer if needed, or open your sleeping bag to cool off and prevent overheating and sweating. The amount of heat your body produces during the night fluctuates and is influenced by factors like how recently you’ve eaten. If you wake up feeling cold, eat something sweet and fatty, like a candy bar or granola bar, to help warm up.
Sleeping warm is a skill
Sleeping warm at night can be achieved by taking steps to ensure comfort, such as wearing loose, dry, long underwear, venting your sleeping bag when you’re too warm, or jumpstarting your metabolism by eating something sweet and fatty if you get chilled.
