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When the temperature drops, nothing will keep you as warm as well as a mummy sleeping bag. Most contain high-quality synthetic fibers or down insulation to help you retain your body heat. The body-tight design minimizes heat robbing dead air space, and many come with smart hoods that cinch tightly around the head and neck, and baffle and zipper features that keep heat from sneaking out or cold from sneaking in. When it’s time to bunk down on a frosty night, here are three things to look for in a mummy sleeping bag.

This product has a specialized zipper that doesn’t bind. Coleman

A mummy bag holds heat, in part, because the form-fitting design reduces the amount of dead air space between your body and the bag. That makes it important to buy the right size bag. Many manufacturers make mummy sleeping bags in sizes for short people and tall folks alike, and there are a growing number of bags tailored for women.

This product has a full-length draft tube along the zipper to prevent cold air from seeping in. TETON

A close-fitting sleeping bag is a great thermal concept, but some people find the confinement a little too much, especially around their feet. If that sounds like you, look for a mummy bag with extra room in the footbox so you can cross and uncross your feet.

This product has a three-piece hood for excellent heat retention. TETON

Most designers engineer mummy bags for cold conditions, but such a tailored cut and lots of lofty insulation can come at the cost of breathability. On average, down is more breathable than synthetic fill materials. So, if you’re a warm sleeper, or will be camping in conditions that hover around the top of a sleeping bag’s temperature range, you might consider a bag filled with down insulation.