Mummy sleeping bags
There are two main sleeping bag shapes. A rectangular-shaped sleeping bag is a more traditional style, while a mummy-style sleeping bag is more form fitting and compact…it actually resembles an Egyptian mummy somewhat. Each type of sleeping bag shape has its own characteristics, strengths, and weaknesses, which we’ll explore in a little greater detail.
The second major type of sleeping bag, sometimes called a mummy bag because of its shape, is different in a number of important ways. It tapers from the head end to the foot end, reducing its volume and surface area, and improving its overall heat retention properties. Some bags are designed especially to accommodate women's body shapes. Most mummy bags do not unzip all the way to the feet, because the zipper is a weak point in any sleeping bag's insulating qualities. Together with the tapered shape, this design feature helps protect the feet, which are more vulnerable to heat loss than other parts of the body.  Another design feature is a drawstring, equipped with a cord lock, at the head end to help prevent the escape of warm air. A mummy bag often cannot be rolled like a rectangular bag. Instead, it is simply stuffed into a stuff sack or compression sack.
The bottom of a sleeping bag typically does not provide significant insulation, because body weight crushes the loft of the insulation material. It is therefore necessary to use a pad or other less crush-able insulation underneath the sleeper, especially in cold weather. Due to this, some sleeping bags do not include insulation on the bottom. Some include a sleeve for holding a sleeping pad. Additionally, some campers, especially ultralight backpackers or hammock campers, have started to use a top quilt, essentially a sleeping bag without a back. Some top quilts include a foot box, while others are just simple blankets.
Sleeping Bag Comfort
Compared to rectangular sleeping bags, mummy bags, by design, offer less room inside which can result in some people feeling constricted. In addition, a person in a mummy bag must usually lie on his or her back for sleep. A rectangular bag, by contrast, offers more freedom of movement and a variety of sleeping positions.
Sleeping Bag Warmth
A mummy bag is typically more efficient in very cold temperatures and, when they’re fully closed, only the mouth and nose of the sleeper are exposed. The form-fitting nature of a mummy bag increases its insulation qualities by reducing the amount of air around the body, so less heat has to be generated to maintain a constant temperature.
Sleeping Bags and Backpacking
Mummy style sleeping bags are lighter and less bulky than comparably rated rectangular bags, making them ideal for backpacking and backcountry camping. Some lighter and smaller rectangular bags can be suitable for short-to medium-distance backpacking in moderate temperature climates. Remember, heat efficiency in rectangular sleeping bags is compromised, due to the more open, non-conforming cut.
Sleeping Bag Prices
Cost often plays into the decision between a mummy bag versus a rectangular sleeping bag, for serious campers. The higher end, expensive, mummy bags can out price the priciest rectangular bags by several hundred dollars, while lower end mummy bags are often similarly priced to most rectangular bags. At the end of the day, let your use, frequency, personal preference, and pocketbook guide your selection.