A sleeping bag traps and holds air around your body.  Your caloric engine warms the air, which is in turn insulated by the sleeping bag.  The fabric on the inside traps the air in, while the outer fabric keeps the outside air out.  These fabric sheets are usually stuffed with some type of material to provide additional insulation and comfort.  Whether you have a $350 mummy bag emblazoned with Timon and Pumbaa, the basic principle is the same.  The Performance, however, will be drastically different.

The two main shapes of sleeping bags are mummy and rectangular. These two types can each be classed in two further categories: down fill and synthetic fill.  So, in essence, mummy/synthetic.

Mummy bags insulate more effectively; rectangular bags permit more movement and comfort.  If you camp only in warm climates, a rectangular bag is you best choice.  If you camp in varying temperatures, it is best to own a rectangular bag for warm weather and a mummy bag for cold weather.  If you need to bring a bag to a camp lodge for a church retreat, get a rectangular bag.

Down filled bags weigh less and provide better insulation than synthetic-filled bags.  They also compress into smaller shapes and tend to be of better quality.  Synthetic-fill bags cost less, and wet synthetic bag works better than a wet down bag.

Temperature or comfort rating describes the minimum temperature the bag is designed for.  A bag designed for 50 degrees is really only suited for comfortable sleeping in warm weather.  A bag rated a 0 degrees is ok for winter camping, but you would sure want to be inside a tent or bivvy sack.  The ratings the manufacturer provides are only a guide.  Some people sleep quite comfortably in a 40 degree bag at 20 degrees, while another person will shiver in 40 degrees weather in a bag rated to 20 degrees.   If you’re the type who needs flannel pajamas and an electric blanket in October, you need to compensate for that. If you sleep in a tiny silk teddy and  ---ahh. Nevermind.