On a cool evening in an unfamiliar place, a good sleeping bag seems to work like magic. Slip inside one after a few post-sundown shivers have rattled your body and, within minutes, the chill in your bones is replaced by a warm glow. It's a sweet sensation that assures you of a comfortable night's sleep.

Here are some tips to help you make a smart choice when selecting your own sleeping bag.

Quick Read

1. Match your bag's comfort rating with the coldest nighttime temperatures you expect to encounter—and maybe even exceed that number for little security.
   
   
2. Bags using down insulation are lighter (providing a higher "warmth-to-weight" ratio) than bags using synthetic fill. They also compress into smaller shapes and last longer.
   
   
3. Synthetic-fill bags can provide some insulation even when wet, and they dry out fairly quickly. Plus, for the same temperature rating, they cost less than down bags.
   
   
4. A bag's shape matters. Mummy-style bags insulate most effectively and are your best choice for colder, high-elevation conditions; rectangular bags give you more room to change sleeping positions but offer more space that your body must heat up.
   
   
5. A good sleeping pad is essential. Your body weight compresses a bag's insulation when you lie on it, so you need a reliable buffer between, your bag and the cold ground.
   
   

How Do Sleeping Bags Work?

Sleeping bags keep you warm by trapping and holding a layer of "dead" (non-circulating) air next to your body. This air, which is warmed by your body heat, forms a barrier between you and colder air or cold surfaces.

When evaluating bags, consider these key factors:

• Comfort rating
• Insulation (down or synthetic fill)
• Weight
• Size when compacted
• Shape
• Personal sleeping tendencies (are you, for example, a "cold sleeper"?)

Comfort Rating

A sleeping bag's temperature or "comfort" rating identifies the most extreme temperature the bag is designed to accommodate. When you hear a bag described as a "-7 bag," it suggests most users should remain comfortable if the air temperature drops no lower than -7 degrees Celsius (° C).

Are such ratings infallible? No. Humans all have different metabolic rates, and no industry standards exist that uniformly determine sleeping bag comfort ratings. Instead, each manufacturer assigns a rating to its bags based on its own research. Therefore, use these numbers as a guide, not a guarantee. If you have trouble deciding between two bags, it's not a bad idea to select one that offers a little more warmth than you think you might need.

Many factors affect your ability to keep warm inside a sleeping bag:

• The insulating pad beneath your bag: When sleeping on frosty ground at high elevation, you need a full-length pad to keep you separated from the cold; when sleeping on snow or frozen ground, two pads are recommended
• The presence/absence of a tent: a tent or bivy shelter traps an extra layer of dead air, warming it by up to 5 degrees
• Your metabolism; you might be a "cold sleeper" - and thus one who prefers extra insulation when sleeping, or a "warm sleeper" someone who kicks the covers off at home
• Your gender: women frequently prefer bags with lower temperature ratings since they tend to "sleep colder" than men
• Clothing worn while inside the bag: dry long underwear and clean socks are good choices on cold nights, plus they help keep body oils off your bag; a cap and neck gaiter keeps body heat from radiating away; fleece pants and jackets help on colder-than-expected evenings
• Adjustments you make while in the bag: keep the bag zipped up and the hood cinched on cold nights; be careful to not breathe into the bag, since moisture has a negative effect on the insulation
• Food in your stomach: the process of digestion helps produce warmth
• Hydration: if you're not well hydrated the food won't help much

Even experienced campers and backpackers can be surprised by unexpectedly cold overnight conditions, particularly during trips in the spring and fall. It's smart to be prepared.

TIP: To be ready for those extra chilly nights, select a bag with a temperature rating that slightly exceeds the low end of the temperature range you expect to experience. If a -7° C bag sounds right for you, a -12 bag would probably work well, too. On warm nights, you can always vent a bag (by using the double zipper to open the area near your legs) or simply drape it over you, unzipped. It never hurts to be a little over-prepared.

Recognizing that comfort ratings are merely general guides here are the popular categories:

Down or Synthetic Insulation?

The insulation or "fill" inside a sleeping bag largely determines a sleeping bag's:

• Weight (and thus its "warmth-for-weight" ratio)
• Compressibility
• Durability

Down
Down is the wispy, fluffy undercoating found just beneath the outer feathers of geese and ducks. This natural fiber is an extraordinary insulator. Goose down is preferred to down from ducks, prized because it is believed its plumes offer a higher "fillpower" (explained below).

Down's positives include:

• It offers tremendous warmth for surprisingly little weight (thus offering a superior "warmth-to-weight" ratio).
• It can be compacted into very small sizes.
• Its effectiveness outperforms synthetic insulation by years—even decades.

Down, though, does have a downside:

• If it gets wet, it is of no value until it dries—and in the field, that can take a long time.
• It is more expensive (keep in mind, though, that its resistance to deterioration makes it an outstanding long-term value).

Down is graded according to fill power—meaning the number of cubic inches one ounce of down will displace. The higher the number, the better the insulation.

Synthetic Materials
Synthetic materials are basically plastic threads (extruded polymers, to be technical). The threads are most commonly a continuous filament (a long, single strand). They can also be arranged in short "staples" up to four inches long. Usually the threads are hollow, reducing their weight and enabling them to trap more air.

The advantages of synthetic fill include:

• It still provides some insulation when wet; plus it dries fairly quickly.
• It's less expensive than down.
• It's non-allergenic.

The shortcomings of synthetic fill are:

• It's bulkier than down (so it takes up more space when you're carrying it).
• It's heavier (it takes more weight to get the same warmth down provides).
• The filaments gradually degrade over time.
• The insulating "batts" of filaments are stiffer than down and do not drape over the contours of your body as effectively.

Which is Right for You?

Down works well for just about everyone except people who frequently find themselves in rainy conditions.

Synthetic insulation is a good choice for kids and newcomers to camping and backpacking. It costs less than down and dries out relatively quickly if it gets wet.

Many women's bags are cut to accommodate a woman's body shape and preference for extra insulation.

Down always wins in terms of weight, compressibility, warmth and durability. Yet the value and performance of synthetic bags makes them very popular. Synthetic bags are improving each new model year, and they're champs when rain is a threat or cost is a factor.