Sleeping bags are designed to keep users dry and warm when sleeping away from the protective cocoon that is their bed. They are lightweight to make them easy to carry, and they prevent sleepers from having to suffer from hypothermia or exposure if bedding down outdoors overnight.
Sleeping bags feature thermal insulation that preserves the body warmth of the sleeper. The material is lightweight and can be rolled into a very small pack that is then stored in a bivy made from waterproof material. An optional feature is the sleeping pad which is rolled up when not in use. It is placed onto the ground---after rocks and larger pebbles have been cleared away---and the sleeping bag is placed on top of the pad. This adds comfort to the sleeping arrangement and an added barrier against heat loss.
There are two types of sleeping bags: the basic square sleeping bag that zippers on the side; and the mummy bag that zippers only partially and is a lot smaller to increase the retention of body heat. Among these types, there are several subtypes, which are differentiated by their temperature ratings and prices. The first kind of sleeping bag is usually seen for backyard camping or sleepovers, while the mummy bag is a more serious survival tool that allows a sleeper to survive even in extreme weather conditions.
Remember that not all sleeping bags are created equal, and carefully check the temperature ratings before committing to the purchase. Additionally, serious hikers and campers often have two of more bags for the various weather conditions they encounter on their hikes and camping events. To complicate matters further, sleepers with higher metabolisms find that sleeping bags rated for withstanding lower temperatures---by preserving more body heat---are uncomfortable as they wake up sweating profusely. Trial and error are usually the means by which to find that perfect bag for the right occasion.
Do not simply rely on a sleeping bag rated for mountaineering temperatures to protect you from the cold. Instead, the sleeping bag will provide the assistive tool to preserve body heat in extreme temperatures, but you must use it with common sense in mind. This means wearing thermal underwear, being dry prior to zipping yourself into the bag, and also using a pad underneath the bag to help with the prevention of heat loss. Misuse of the bag may result in an uncomfortable night or serious health problems due to frostbite.
The sleeping bag has served as the model for the infant sleeper, also known as a sleeping sack or infant sleeping bag. Due to the danger of sudden infant death syndrome (SIDS), parents are strongly discouraged from using blankets in the cribs of infants. To keep the babies warm in cold climates, the use of a zippered sleeping sack that features arm holes to keep it in place--and away from the child's face---is now very popular. The child's body is cozy and warm inside the sack, while she is not in danger of suffocation.
