A good sleeping bag is one of your more important pieces of camping equipment; you might as well do some research before you buy one, because they can be quite expensive. First off, there are several important things to consider when browsing:

Intended use(s)
If you are just going to use your bag for slumber parties and other indoor activities, by all means go ahead and buy one of those El Cheapo Grande cotton sleeping bags at your local sporting goods store. They're great for just mucking around in and if you spill beer all over yours at some shindig you can usually just toss it in the washing machine. However, if you are plannning on climbing K2, you'll probably want something a little more heavy duty.

Since bags are rated by temperature, consider what seasons you'll be using yours during. If a bag is rated to 30 degrees, this generally means it will effectively keep you toasty warm until the mercury hits 30 degrees Fahrenheit, or thereabouts. For winter camping, you'll want a bag that is good until a temperature that is lower than the expected maximum low for where you're going. (You can always bring along a liner or a sheet to put inside your bag; these will usually give you about an extra 10 degrees of comfort.) For most people, what is known as a "three season" bag is sufficient.

If you are a backpacker or biker and concerned about weight, you may want to consider buying a bivy sack, which is a combination of tent and sleeping bag, usually with a waterproof bottom and some sort of fly over the head. Try before you buy, if possible; some people find bivys to be a bit suffocating. They are a bit more costly than regular bags, depending on the brand you get. Army/Navy surplus stores sometimes sell them at a considerable discount.

Size and shape of both you and your bag
Sleeping bags come in several different shapes. There's your basic rectangle, (self-explanatory; seen often in the El Cheapo Grande bags mentioned above and in that He-Man bag you had when you were little) the barrel, (rounded at the edges, a little wider in the middle) and the mummy (relatively narrow, usually with a hood-type thing that you can pull tight around your head.) Rectangular bags are good for restless sleepers because they give you a lot of room to move around in. Barrel bags are basically just a variation on a theme; sometimes they pack up smaller than rectangular bags. Mummy bags are probably the most popular, as they take up very little of your valuable backpack space. However, some people complain that mummy bags are constricting. Personally, I prefer a mummy bag because of the nifty hood, which comes in handy when you're pushing your bag's temp. rating. (You lose most of your body heat through your head, so wear a hat or hood on a cold night.)

Now, consider the shape of yourself. No, not your fitness level. Are you tall? Small? Overweight? There are bags out there designed for your needs. A small person may be able to get away with buying a child-sized bag, but make sure it has what you're looking for. Also think about how you sleep. Some bags have extra room for people who kick their feet, others have dual zippers so you can zip two together and make a wilderness love nest.

Your budget
A bag from Expensive Mountain Shit (oops, make that Eastern Mountain Sports) will, not surprisingly, probably be more expensive than one you find at your neighbor's garage sale. Good deals on camping equipment can usually be found in discount catalogs like Campmor (www.campmor.com) and Sierra Trading Post, and in the L.L. Bean sale catalogs. Some stores (like EMS) will let you rent various items so you can take them on a weekend trip into the backcountry and put them through their paces. Go with whatever works best for you, but, as with all your other equipment, never compromise your comfort and safety.

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