There is no industry standard method of determining temperature ratings of sleeping bags. 30 o F sleeping bag. Given the imprecision of manufacturers' ratings and given the wide variation in people's comfort levels, it is advisable to consider bags within a range of temperature ratings. Very space and weight efficient because their tapered shape reduces the amount of fabric and fill that must be packed and carried. The lack of dead space allows these sleeping bags to warm up more quickly and keep you comfortable in colder temperatures than does a bag with a less tapered cut.
Some people find the cocoon-like feel comforting, a few find it claustrophobic.
The whole point of a sleeping bag is to let you sleep comfortably. If a mummy is too constraining, a few more ounces of carrying weight and a few more cubic inches of sacrificed pack space is a reasonable price to pay for a good night's sleep. However, because rectangular bags are more difficult to warm up and bulkier, they are best suited for summer camping and car-camping. A semi-rectangular sleeping bag sacrifices some thermal efficiency and pack size for extra wiggle room. Though some mummies are designed so they can be coupled, semi-rectangular bags often provide a more natural fit when two people wish to zip their bags together and share body heat.