Your sleeping bag and your wind sack together form the most important piece of equipment from a safety perspective when you are out in the winter mountains. Taking care of your sleeping bag, therefore, should be a top priority. Make sure that the down or padding is not falling out and preferably store it in a waterproof roll-top bag.
What is most important is protecting your sleeping bag from moisture since moisture conducts cold and impairs the bag’s ability to insulate. For this reason you should never try to dry clothes or shoes in your sleeping bag (an old “trick” that is sometimes mentioned when talking about life in the high mountains). Instead, take advantage of every opportunity to dry out your sleeping bag before your pack it away, during your trek and when you are about to crawl into it. This is particularly important if you will be using your sleeping bag over a long period of time.
Experienced outdoors people frequently opt to bring two sleeping bags on their winter trek - one thin down sleeping bag and one thin synthetic sleeping bag - instead of a single thick sleeping bag. The synthetic sleeping bag is used closest to the body. When your body releases moisture, this moisture is absorbed first by the less sensitive synthetic sleeping bag, which means the down sleeping bag stays in good condition longer.
More practical tips:
• Have a thick, insulating ground pad under your sleeping bag.
• Shake out your sleeping bag so the padding achieves maximum loft.
• Make sure you are warm before you crawl into your sleeping bag – drink something warm and hop around for a while if you need to make yourself warmer.
• Wear as little as possible in your sleeping bag - a thin (and dry) base layer is enough. If necessary, pile reinforcements - such as a jacket - on top of your sleeping bag.
• Try not to sweat.
• Take a warm water bottle into the sleeping bag with you. This will help raise the temperature and drive out moisture. The bottle should have protective wrapping, for example a wool sock or a neoprene cover, so it is not too warm against your skin.
• Air the backpack every day to remove moisture.
• Sun and wind dry out your sleeping bag, so take advantage of good weather.