In camping and other outdoor activities, a sleeping bag is a protective "bag" for sleeping, analogous to a bed. Its primary purpose is to provide warmth and insulation. It also protects, to some extent, against wind, precipitation, and exposure to view, but a tent performs those functions better. The bottom surface also provides some cushioning, but a sleeping pad is usually used for that purpose. A bivouac sack (bivy) is a waterproof cover for a sleeping bag that may be used in place of a tent, if rain or very cold temperatures are not expected.
A basic sleeping bag is simply a square blanket, filled with cotton or other material, and fitted with a zipper on three sides allowing it to be folded in half and secured in this position. A sleeping bag of this type is packed by being folded in thirds, rolled up, and bound with straps or cords. The basic design works well for most camping needs, but is inadequate under more demanding circumstances.
The second major type of sleeping bag, sometimes called a mummy bag due to its shape, is different in a number of important ways.
- It is tapered, with the foot end narrower than the head end. This reduces both volume and surface area, improving its overall heat retention properties.
- It usually only unzips part of the way, so that the foot end is permanently cylindrical in shape. The zipper is a weak point in any sleeping bag's insulating qualities. Together with the tapered shape, this design feature helps protect the feet, which are more vulnerable to heat loss than other parts of the body.
- It usually has a drawstring mechanism at the head end, to help prevent the escape of warm air.
- The most important difference is the insulating material. "Mummy" bags are filled with synthetic, hollow fibers. This type of fill does not readily absorb water, dries easily, and will provide some warmth even when completely wet. These properties may be a matter of life and death if, for example, the sleeping bag is accidentally dropped into a river on a cold day. A soaked cotton sleeping bag may provide even less insulation than no sleeping bag at all, leading to hypothermia. Also, the synthetic material is highly resilient to compression. Any insulaton is compromised when it is compressed, so this makes the sleeping bag warmer on the bottom.
