Mummy bags are lightweight and designed for optimum warmth, easy to carry, and cozy when inside. They can be somewhat restrictive when it comes to movement, as they are designed to follow the contours of the body for a snug fit. This can be inconvenient to larger users or those that are restless sleepers. Rectangular bags provide more wiggle room, though they are often bulkier, which may not convenient when on a long trek into the mountains.