A basic sleeping bag is simply a square blanket, fitted with a zipper on one or two 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 half or 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 because of its shape, is different in a number of important ways.
Many different insulating materials are available for sleeping bags. Outdoor professionals usually prefer either synthetic fill, such as PrimaLoft, or down, and they have debated the merits of these materials for years.
Synthetic fill does not readily absorb water, dries easily, and provides some warmth even when thoroughly soaked. These properties may save the owner's life if, for example, the sleeping bag is accidentally dropped into water on a cold day. Synthetic material is also firm and resilient, so it insulates well even underneath a person's weight. Synthetics also have the ability to loft faster than down, allowing the sleeping bag to provide the insulation faster than a down bag. On the flipside, synthetic fill cannot be compressed as much as down, causing such bags to take up more space when not in use. Furthermore synthetic insulation tend to break down faster than their natural counter part.
Down fill weighs less than synthetic and retains heat better, but usually costs more. Down must be kept dry; a soaked down sleeping bag may provide even less insulation than no sleeping bag at all, leading to hypothermia. Newer, more technically advanced sleeping bags often have water-resistant shells and can be used in damper conditions. It is also recommended to keep a sleeping bag in a larger sack (storage sack) as opposed to the small traveling sack (compression bag) during long periods of storage. However, many regular backpackers and hikers agree that hanging a sleeping bag, taking care to move the position of the bag on the hanger at intervals so as to not create a "dead spot" (a spot where the fill has been crushed so that it is no longer useful), is the best method of storing a bag for long durations.
Other materials, notably cotton and wool, have also been used for sleeping bags. Wool repels water nicely and also resists compression, but it weighs much more than any alternative. Cotton suffers from high water retention and significant weight, but its low cost makes it an attractive option for uses like stationary camping where these drawbacks are of little consequence.
The transition zone, in between the comfort and lower temperature, is usually considered as the best purchase guideline.
Indoor sleeping bags, sometimes called slumber bags, are widely available, often for use particularly by children. These are usually not designed to be weatherproof, and are often made of natural fabrics instead of the synthetic fabrics commonly used for outdoor sleeping bags. Children's sleeping bags in particular often feature elaborate, brightly-colored printed designs, such as images of popular media characters. Slumber bags make floor sleeping more comfortable, and are often used for sleepovers, family visits, and other situations where there are not enough beds for everyone.