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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 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.
The "Euklisia Rug", patented by mail-order pioneer Pryce Pryce-Jones in 1876, is considered by many to have been the first forerunner of the modern sleeping bag. Pryce-Jones, a Newtown, Montgomeryshire Welsh entrepreneur developed the bag and exported around the world in the late 19th century. Documents show he sold 60,000 of these rugs to the Russian army - and the British army also bought them. There are records of civilian uses too - among missionaries in Africa and pioneers in the Australian outback. 
A basic sleeping bag is simply a square blanket, fitted with a zipper on two or 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 half or thirds, rolled up, and bound with straps or cords with cord locks. 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. Most mummy bags do not unzip all the way to the feet, because 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. Another design feature is a drawstring, equipped with a cord lock, 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.
The bottom of a sleeping bag typically does not provide significant insulation, because body weight crushes the loft of the insulation material. Due to this, it is necessary to use a pad or other less crush-able insulation underneath the sleeper, especially in cold weather. Due to this, some sleeping bags do not include insulation on the bottom. Some include a sleeve for holding a sleeping pad. Additionally, some campers, especially ultralight backpackers or hammock campers, have started to use a top quilt, essentially a sleeping bag without a back. Some top quilts include a foot box, while others are just simple blankets.
Many different insulating materials are available for sleeping bags. Outdoor professionals usually prefer either synthetic fill (e.g. PrimaLoft), or natural fill (e.g. 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. On the flipside, synthetic fill cannot be compressed as much as down and it weighs more, causing such bags to take up more space and weight when not in use. Furthermore synthetic insulation tend to break down faster than its natural counterpart.
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 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, you 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. And there are other variables to consider, such as what you plan to wear when you sleep, what type of sleeping pad you use, and how well you hold the heat in your 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."
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