Which sleeping bag feature can restrict movement but provide more warmth?

The mummy design of a sleeping bag is specifically engineered to conform closely to the user's body shape, which minimizes the amount of air that can circulate within the bag. This close fit allows the sleeping bag to retain heat much more efficiently compared to other designs, helping to keep the user warm in colder temperatures. Additionally, the tapered shape of the mummy bag reduces weight and bulk, making it a preferred option for hikers and backpackers looking for both warmth and portability.

While a rectangular shape offers more freedom of movement, it tends to leave more space for cold air to circulate, making it less effective in retaining warmth. An open design obviously lacks any kind of constriction or insulation that comes from being wrapped closely, leaving the user exposed to the elements. Down insulation is a material choice that primarily affects warmth but does not inherently restrict movement. Thus, using the mummy design provides the desired combination of warmth and restricted movement that is beneficial in chilly conditions.