Choosing a Sleeping Bag

A good sleeping bag is considered a critical piece of equipment on almost any backpacking trip. A quality sleeping bag will keep you warm, dry, and well rested in spite of the weather outside. Proper rest and recovery will go a long way to make sure that your body can handle the rigors or backpacking. Consider the following factors when choosing a sleeping bag:

Comfort Rating -
How cold can it get before you start to feel it? A sleeping bag's comfort rating is supposed to designate the most extreme temperature that the bag will accommodate and still keep an average person comfortable. So when you see a bag that is rated to 30 degrees, it means that the average person would be comfortable in the bag as long as the temperature stays above 30 degrees.
It should be noted that these ratings are not absolute. Not only are they unable to account for subjective sleeping attributes of the individual, but there is also no standard for determining them. One manufacturer's 20 degree bag might be another's 30 degree bag. Therefore, these numbers should be used as a guide. If you're choosing between bags, it's not a bad idea to go with the one that offers more warmth. Remember, you can always unzip the bag if you get too warm.

Warmth Factors -
There are also a number of factors that will determine how warm you are in your sleeping bag.

Sleeping pad - when you use your sleeping bag the fill directly under your body is compressed and loses much of its insulating ability. A sleeping pad insulates you from the cold ground and also provides additional comfort.
Tent - If you use a tent you'll be adding another layer or dead air space around you that will keep you warmer over all than if you slept under the stars. Keep in mind that the effectiveness of this factor will vary depending on the particular design of your tent.
Gender - Women often prefer warmer bags than men. This can also have an effect on how a bag fits.
Personal Sleeping Habits - Do you sleep hot or cold? Are you one to kick off the covers or do you wake up unable to move because of all the blankets you've wrapped up in?
Food & Hydration- Having something on your stomach when you hit the sack will keep you warmer. The process of digestion generates warmth. Hydration is also important too, since the food won't generate much warmth if you are dehydrated.
Clothing - The clothing you wear in your sleeping bag has a large impact on how warm you are. There is an urban legend that you will often hear that you will be warmer in the nude. Though this certainly makes things more interesting for your hiking buddies, this is not the case. Wearing clean and dry long underwear with socks will help you stay toasty on cool nights. On colder nights, a cap and neck gaiter will help keep body heat from radiating away.

Weight - All things being equal, lighter is better. Some fills are lighter than others. Generally you will pay more for fills that offer better warmth to weight ratios. Down will offer the lightest weight for the most amount of insulation. Synthetic fills are heavier for their insulating ability.

Personal Sleeping Habits - Do you sleep cold or hot, toss and turn, etc? If you are one that moves around in your sleep, there are bags available that offer elasticized seams to keep the bag close to your body. Make sure that your bag accommodates your personal sleeping tendencies. After all, the idea is for you to be comfortable, right?

Packing Size - Because no one likes to carry a Volkswagon. Again, some fills offer more compressibility than others. Make sure that when you are in the store that you pack the bag as you would while on the trail. This will give you a good idea as to how bulky the bag really is when you pack it.

Fit - If it's not comfy, what's the point? Having a good fit is more than just comfort, though. If your bag doesn't fit, you can lose up to 15 degrees off of the comfort rating. It should be snug, but not constrictive. Sleeping bags often come in regular and long sizes for those of us that duck through doorways. If you're a larger framed person, you might want to look at a modified mummy design on your bag. The modified mummy is a cross between a mummy and a rectangular bag and offers more girth and wiggle room for a larger person.
Some manufacturers also offer bags designed specifically for women. These bags will often have more room in the hips and be narrower in the shoulders. They also will have more fill across the torso and in the footbox where many women seem to get colder. Always try the bag for fit while you're in the store and make sure to try more than one so that you can compare. The trail is no place to find out that your sleeping bag is too small.