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A sleeping bag is a protective "bag" for a person to sleep in, essentially a blanket that can be closed with a zipper or similar means, and functions as a bed in situations where it is impractical to carry around a full 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 for lightweight travelers or as a backup if inclement weather occurs.

A mummy bag

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.

• It tapers from the head end to the foot end, reducing its volume and surface area, and improving its overall heat retention properties. Some bags are designed specially to accommodate women's body shapes.
• It usually does not unzip all the way to the feet. 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 at the head end, to help prevent the escape of warm air.
• A mummy bag often cannot be rolled like a rectangular bag. Instead, it is simply stuffed into a stuff sack or compression sack.

A highly compact sleeping bag measuring 23 cm with a diameter of 12 cm when packed but 210 cm x 65 cm when unfolded.

Many different insulating materials are available for sleeping bags. Outdoor professionals usually prefer either synthetic fill 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.

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.

A person in a sleeping bag

In Europe, the EN 13537 standard normalizes the temperatures at which a sleeping bag can be used. A test, relying on a heated mannequin, provides four temperatures:

• the upper limit is the highest temperature at which a 'standard' adult male is able to have a comfortable night's sleep without excess sweating.
• the comfort rating is based on a 'standard' woman having a comfortable night's sleep.
• the lower limit is based on the lowest temperature at which a 'standard' adult male is deemed to be able to have a comfortable night's sleep.
• the extreme rating is a survival only rating for a 'standard' adult woman. This is an extreme survival rating only and it is not advisable to rely on this rating for general use.

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 aren't enough beds for everyone.

Manufacturers of sleeping bags include:

• Butler Bags
• Big Agnes
• Carinthia
• Coleman Company
• Haglöfs
• Kelty (company)
• Lafuma
• Mammut
• Marmot
• Wiggy's
• Mountain Hardwear
• Snugpak
• Taiga Works
• The North Face
• PHD Mountain Software
• Western Mountaineering
• Eastern Mountain Sports [[1]]
• Feathered Friends

• Sleeping bag (infant)
• Pertex

• Sleeping Bags How to select a sleeping bag.
• Sleeping Bag Selection A primer on sleeping bag selection, care and use.
• Washing sleeping bags Information on cleaning and caring for down sleeping bags.
• BackpackGearTest.org Backpackers evaluate new gear in the field and report their findings