I have always heard that the best way to stay warm in a sleeping bag is to zip up with no clothes on. I've been told by both military and civilian that sleeping naked in a sleeping bag is warmer than wearing long underwear inside of the bag. I once overheard a surplus store employee give advice on sleeping without clothing to a customer. My findings through experience? That "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 from the heat of your body. The idea is to create a barrier or layer between you and the outside air or ground. More clothing translates into more layers of air that are created around your body. The more layers of warm air you create around your body, the warmer you'll be. When you wear long underwear in a sleeping bag, you increase the amount of layering insulation between your skin and the cold air outside. This will make you much warmer than if you sleep naked.
Is it ALWAYS true that more clothing will keep you warmer?
Keep in mind that air is trapped in the gaps or spaces in the insulation of the sleeping bag. If you are wearing too much clothing or have too much extra gear stuffed inside the bag you can compress the bag's insulating layer. This significantly reduces the amount of warm air that can be trapped.
Be careful of super-tight fitting long underwear and socks because they can reduce blood circulation to your extremities. Reduction in blood circulation makes you colder.
Wet clothing compromises the insulation in your sleeping bag as your body heat dries it. The moisture in your clothes gets trapped by the sleeping bag’s insulation which significantly degrades its insulating effectiveness. Wet does not always mean "I fell in the river and am soaking wet" wet. Even a small amount of sweat can greatly reduce the effectiveness of your sleeping bag's insulation. The best practice to wear clean and dry long underwear and socks inside your bag.
Speaking of sweat
Wearing too many clothes in your sleeping bag can cause you to sweat. Sweat, as it dries, significantly degrades the insulation in your sleeping bag, just like wearing wet clothing. To manage, simply remove a layer or unzip your sleeping bag to reduce the buildup of heat.
What I wear in my sleeping bag
I use one of two sleeping bags. One, the MMSS, Military Modular Sleeping System 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 beenie or watch hat). The clothing not only provides additional insulating layers but also keeps the inside of my sleeping bag clean, and that in turn keeps me warmer at night. I have at times slept in my coat and insulated pants inside of my sleeping bag and was able to keep my warmth well within my personal comfort zone. At times I will 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 of your bag is unpleasant and reduces the effectiveness of the sleeping bag's insulation. Remember to remove a layer if needed and 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 how recently you ate food. Eat something sweet and fatty like a candy bar or granola bar if you wake up feeling the chill.
Sleeping warm is a skill
Sleeping warm at night can be achieved by the steps you take to sleep comfortably like wearing loose, dry, long underwear, venting your sleeping bag when you’re too warm, or jumpstarting your metabolism by eating something sweet and fatty when you get chilled.
Sleeping Bags and Sleeping Gear to Buy (Amazon Affiliate)
- USMC 3 Season Sleeping Bag
- USMC Extreme Cold Weather Bag for 3 Season System
- Military Modular Sleep System sleeping bag
- Popular Sleeping Bags Popular Long Underwear
- Ultimate Wilderness Gear: Everything You Need to Know to Choose and Use the Best Outdoor Equipment: Buy it on Kindle or Buy it in Paperback