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 a bed is unavailable (e.g. when camping, hiking, hill walking or climbing). Its primary purpose is to provide warmth and thermal insulation. It also protects, to some extent, against wind chill, 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 in addition 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 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 man is able to have a comfortable night’s sleep without excess sweating.
- The comfort rating is based on a ‘standard’ adult woman having a comfortable night’s sleep.
- The lower limit is based on the lowest temperature at which a ‘standard’ adult man is deemed to be able to have a comfortable night’s sleep.
- The extreme rating is a survival only rating for a ‘standard’ adult man. 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.
A sleeping bag’s rating typically indicates the lowest temperature at which it will keep the average sleeper warm. For example, with a 0° bag, a person should be able to sleep in 0° temperature, but not necessarily comfortably.
There is no standard measurement rating in the U.S., so a 20° bag from one company may not provide the same warmth as a 20° from another company. Other important variables include what the user plans to wear while sleeping, what type of sleeping pad is used, and how well the user holds heat in the bag.
Indoor sleeping bags
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.
An infant sleeping bag is a bag-like garment or covering worn by infants for sleeping in. Infant sleeping bags differ from regular sleeping bags in design and purpose, being designed primarily for indoor rather than outdoor use, and usually featuring either arm holes or sleeves.
The definition used in the British Standard for safety of children’s sleep bags is “sleep bags for the use of children with a minimum weight of 4 kg designed to provide sufficient warmth so as to remove the need for additional bedding when sleeping in a cot or similar product in which a child is contained.” It goes on to exclude “garments with sleeves and feet, i.e. sleep suits or baby gros, or to products designed primarily for outdoor use or to keep children warm when in a pushchair.