The function of an sleeping bag is to protect and warm a person who is sleeping.
A sleeping is used as bed by campers. Normaly the sleeping bag is used in a tent because a tent isolating the person much more better from the weather than only the sleeping bag on it self. The sleeping bag can be opened and closed by an zipper.
The sleeping bag is made of few of sintetic materials which were specialy developed for this use. the advantages of this sintetic materials are : they are keeping the person try from water and isolating the person from the weather.