Which type of sleeping bag shape is considered the most efficient in design?

The mummy shape of a sleeping bag is considered the most efficient in design due to its streamlined shape that closely conforms to the body's contours. This design minimizes excess space within the bag, which significantly reduces the amount of warm air that can escape. The tapered foot area and hooded design help trap heat, making it ideal for colder conditions.

In addition to heat retention, the mummy shape is often lighter in weight and more compact for packing, making it easier for activities like backpacking. This efficiency in design translates to better thermal efficiency compared to other shapes, especially when the weather turns chilly, as it requires less insulation material to keep the user warm.

The other bag shapes, such as rectangular or square, provide more space and comfort but are less efficient in retaining heat because they leave more vacant air pockets. Semi-rectangular bags offer a compromise between space and warmth, but they still don't perform as well as the mummy design in terms of thermal efficiency, especially in colder environments. Thus, the mummy sleeping bag stands out as the best choice in terms of functional design for warmth and efficiency.