“There is no such thing as a cold night's sleep, only not enough layers,” he says. “I layer when I'm inside the bag just as much as I do while outside the bag. When you're climbing Everest, you're not naked under your down suit. The more heat you can preserve in a warm layer next to your body, the better.”

There is a common outdoor myth that you should sleep naked or in very few clothes to stay warmer in a sleeping bag, but this is physically incorrect. Modern sleeping bags work by trapping a layer of air that is warmed by your body heat; therefore, wearing dry, warm layers provides an additional barrier that helps retain that heat closer to your skin. The most effective strategy is to wear a clean, dry set of thermal base layers (like merino wool or synthetic silk) and a pair of dry socks. However, there are a few important caveats: you must avoid wearing "too many" layers that compress the insulation of the sleeping bag itself, as the "loft" of the bag is what actually creates the warmth. Most importantly, you should never wear damp or sweaty clothes from your day of hiking into your bag, as the moisture will pull heat away from your body and make you dangerously cold. If you find yourself too warm, it is better to unzip the bag slightly to vent rather than removing layers. In summary: wearing dry, loose-fitting warm clothes is the best way to extend the temperature rating of your sleeping bag and ensure a comfortable night's sleep in the backcountry.