Which sleeping bag shape is considered most efficient for warmth?

The mummy sleeping bag shape is considered most efficient for warmth due to its design that closely contours the body's shape. This snug fit minimizes air space within the bag, which in turn reduces the amount of warm air that can escape. The tapered form of a mummy bag, being narrower at the feet and wider at the shoulders, allows for better insulation around the body's core. Additionally, many mummy bags are equipped with features such as hoods, draft collars, and neck baffles, which further help trap heat.

In contrast, other shapes like rectangular and square bags typically have more excess space and can allow for greater heat loss, making them less efficient in colder conditions. A custom bag might be tailored to an individual's specifications but doesn’t inherently guarantee warmth efficiency without the overall design considerations that benefit the mummy shape. Therefore, the unique design and features of a mummy sleeping bag make it the best choice for retaining warmth in cold environments.