Buying sleeping bags can be a difficult task, with prices ranging from a few dollars to a few hundred. Ultimately you have to ask what you want to use it for.
The sleeping bag is an important part of your camping gear. If you are hiking, then you want something that will pack tightly and is very light weight, although you have to be careful as these sometimes don't provide the best insulation.
When in the wild, temperatures drop at night more than most people expect, even in the middle of summer, so you should make sure you're prepared. A sleeping bag traps the warm air around your body which provides the best insulation possible. The best sleeping bag is one which allows moisture to escape while keeping warm air trapped inside.
If you are in a couple, you might prefer a double sleeping bag (there is no better central heating than the body heat of someone else.)
Sleeping bags are given a temperature rating based upon their intended use. Warm Weather Bags (suitable for 32F and above) are the most popular as most people camp in summer and avoid freezing nights.
Cold Weather Bags (suitable for 10F and below) are for winter and spring camping when temperatures dip below freezing. Don't underestimate the ability of the weather to change unpredictably so always be prepared for temperatures colder than expected. These are also the best bags for survival and emergency use.
Three Season Bags will provide warmth in temperatures which are slightly sub zero and are designed to be a "best of both worlds" type of solution. These are very popular bags.
Temperature ratings should be taken only as a general rule. Remember that temperatures can drop below zero in spring, particularly at higher altitudes so you should allow for weather variables when selecting a bag. The amount of warmth that different people will need can vary widely, with some people requiring more warmth than others. You should also consider if you will have a heater or some other protection from the cold.
What you sleep on is also very important. A good ground mat provides great insulation and is very important when camping.
If you are just planning on camping in the summer, you probably don't need to spend too much, if you are looking at trecking, climbing, mountaineering etc then you really want to look at the top end, as the cheaper models simply won't keep you warm enough. You might also want to consider a "bivvy bag", as this can be useful in emergencies, or if temperatures drop lower than expected.
The fill in sleeping bags can be made of a wide range of things, most commonly goose, duck or down. Down is the warmest and often most expensive but not very good if it gets wet. Newer synthetic fills are often a better choice than down for demanding situations.
You may also want to look out for a bag with a 2 way zip so you can stick your feet out the bottom and double sleeping bags are also available as well as single bags that can join together to form a double.
