The zzzzz zone

You might not care exactly what critter’s feathers are inside your sleeping bag, but that detail can mean the difference between a cozy night’s sleep and waking up with frozen toes.

That’s if you choose to go the feather route. Solving the down versus synthetic-fill dilemma depends on where and when the bag will be used -- and, of course, if you are allergic to the plumes.

Down is superior for its warmth, light weight and longevity. But it’s more expensive and becomes a heavy, slow-drying sponge when wet. A synthetic bag insulates even when wet, and dries quicker.

But in the long-run, down may be a better value than synthetic.

“The front-end cost of a down bag is two to three times greater, but the longevity -- with reasonable care -- will be 10 to 12 times greater,” says John Cooley of Santa Rosa-based Marmot Mountain Ltd., which manufactures sleeping bags. (A survey of sleeping bags on the Marmot website, www.marmot.com, found down bags priced from about $140 to $510, synthetics from about $70 to $130.)

Several down-wannabe synthetics, such as Polarguard and PrimaLoft, have loft -- the ability of a filling to puff up after being pulled from a stuff sack -- nearly comparable to down. Over time, resins in the synthetic material wear off and wash away, lessening the synthetic’s ability to insulate, says Wade Woodhill, product director of equipment for the North Face, another sleeping bag maker.

If you choose a down bag, its fill-power rating -- the amount of pure down per cubic inch -- is a good measure of quality. A 700 rating means 1 ounce of fill will take up 700 cubic inches of space. A higher number means the fill has less debris such as prickly feather quills, more down feathers and, as a result, more loft, warmth and compressibility. These numbers range from a low-end 550 to a rare (and expensive) 900.

If a bag is simply labeled “down,” it’s probably a mix of duck and goose, says Shauna Linde, a spokeswoman for the International Down and Feather Laboratory in Salt Lake City, which runs tests to determine whether a “90% goose down” label on a sleeping bag is accurate.

Most bags end up with a mix of feathers and down clusters. The darker duck feathers don’t insulate as well as down, and some say goose is lighter and packs more poof.

The down that ends up in your bag most likely was once attached to a goose on a supper table an ocean away. Europe and Asia are the primary suppliers of down. “Without goose pate, we wouldn’t have down bags,” Cooley says.

Choosing the right comfort rating can also make a difference. Try to figure out the coldest temperature the bag will have to withstand, then buy a bag with a corresponding comfort rating.

A 20-degree bag, for example, should theoretically keep a person comfortable when the temperature drops to as low as 20 degrees Fahrenheit. People who have trouble keeping warm when they sleep should choose bags with a rating 10 degrees lower than the expected temperature. Minor considerations can also make a difference. Sturdy zippers and bags with hoods to help trap warmth add to the price, but are worth it. Mummy-shaped bags fit more snugly and trap warm air more effectively than rectangular versions, which feel less like straitjackets but are heavier and have more dead space to keep warm.

Also be aware of a sack’s stuffability. There’s nothing worse than plunking down $200 on a new sleeping bag and finding it doesn’t fit in your new $300 backpack.

Regardless of fill and rating, a sleeping bag on its own won’t provide warmth. It traps air that the body radiates. So don’t laugh when your camping partners do jumping jacks before hitting the sack.

“If a body isn’t warm, the bag isn’t going to warm you up,” says Heidi Andersen, a manager at the Adventure 16 store in Tarzana.

The best test of a sleeping bag? Roll it out in the store and spend some time inside it. It may even pay to take a nap. And if the salesperson dares wake you, demand a discount.

To e-mail Julie Sheer or read her previous Outdoors Institute columns, go to latimes.com/juliesheer.