Function

• Mummy bags, like all sleeping bags, are intended to keep their occupants warm and comfortable. As the Sleep Like the Dead website notes, where mummy bags excel is in environments of extreme cold, particularly in subzero temperatures. Their tapered design allows them to fit more securely around the body, thereby conserving maximum body heat. Their design also allows them to be closed so that only the nose and mouth are exposed, ensuring that as little heat escapes as possible.
  
  

Advantages

• There are two main advantages to mummy bags versus rectangular sleeping bags. The first is warmth. While there are some rectangular bags that remain effective in subzero temperatures, mummy bags are specifically designed for such use. The second advantage is weight. Mummy bags are almost always lighter and easier to carry than rectangular bags, simply due to the fact that they're smaller by design. Less total material means less to pack and thus, less to carry.
  
  

Disadvantages

• The main disadvantage of a mummy bag is space. Due to their tapered design, mummy bags require occupants to sleep on their back and offer very little maneuverability beyond that position. This can be a problem for people who are prone to claustrophobia. A second disadvantage is price. A high-end mummy bag can cost several hundred dollars more than a comparable rectangular bag, which leads many people to opt for cheaper alternatives.
  
  

Material

• Like all sleeping bags, mummy bags are offered in both synthetic and down-fill varieties. Synthetic bags are almost always cheaper than down, but they also wear out faster and need replaced more often. Down has the advantage of being significantly warmer than synthetic filler, but it is also more prone to moisture damage. Down bags are also lighter than synthetic bags, which makes them easier to pack and carry.
  
  

Average Cost

• Mummy bags can run the gamut when it comes to prices. Cheaper ones can go for between $40 and $60, while high-end versions can sell for upwards of $500. Bags using down filler as opposed to synthetic will cost more because the down itself costs more, but they will likely need to replaced less often than their synthetic cousins. Apart from filling, the main reason mummy bags can cost so much more is that they're simply more effective.