Sleeping bag shapes are designed to strike a balance between comfort and performance, taking into account the priorities of the users. The bigger a bag is internally, the more room there is to move but the more air there is to heat. Any increase of space in a bag allows more wriggle room, but also reduces the bag’s thermal efficiency.
Sleeping bags come in three shapes:
The most thermally-efficient sleeping bag shape is the mummy. This design is wide at the shoulders with a pronounced taper to a narrow foot, reducing the amount of space – and air to be heated – within the bag. These bags offer maximum warmth for minimum weight; perfect for the adventurer looking for a compact, technical bag for their outdoor pursuits. The narrower fit reduces the ‘wriggle room’ within the bag, which some people may find restrictive. Try one out before you buy, particularly if you’re claustrophobic or like to thrash around in your sleep! Many mummy-shaped bags have half- or three-quarter zips, designed to finish at the knee or calf, reducing the bag’s weight and increasing its warmth. However, a full-length zip gives more versatility as you can open the bag up and vent your feet in warmer conditions, and it also allows easy entry and exit.
Tapered rectangular bags are what they say…They’re less fitted than a mummy bag, but taper to the foot, reducing dead space and the air to be heated within the bag. This style of bag is a good all-round choice as they are comfortably roomy – and can be fully unzipped to make a quilt for your bed – while still being efficient enough to pack down, not tip the scales and keep you warm.
Rectangular bags are roomy and comfortable, giving plenty of space for moving around. Designed for uses where warmth, packed size and weight are not critical, they are generally larger, heavier and less technical bags.