There are two primary issues when choosing a back packing sleeping bag: comfort rating and fill material. The comfort rating is a guide to compare the insulation of sleeping bags. On your trip-determine the lowest temperature that you are likely to encounter, and get a bag that is slightly warmer. 

Don’t forget that higher altitudes are generally cooler. Most spring, summer, and fall hikers enjoy using a rating between 20 degrees and 40 degrees. Winter Hikers may need bags rated well below zero degrees. During the warm summer, it is simple to open the double zipper around the legs for venting, or just leave the whole side unzipped. Sleeping pads greatl;y increase the effectiveness and comfort of a bag by raising a hiker off the cold ground. Mummy bags are always warmer (and lighter) than a bag with the same rating of a rectangular configuration. The fill material is a matter of trade offs and advantages. Down Insulation is warmer, easily compressed, lighter weight, and very durable. Quality down has a fill rating of 500 or higher. The synthetics are less expensive, warm when wet, dry very quickly. Good synthetic insulators are are variations of Polarguard (HV, 3D, v II), Quallofill, and Thermoloft. Most sleeping bags come in two lengths: regular for 5’ to 6’, and long for 6’ to 6’6". Some bags even have a choice of left or right side zippers.