Older sleeping bags were little more than a cotton and wool blanket folded over and fitted with a zipper. Modern sleeping bags have benefited from a dozen advances in materials science and ergonomic design, not to mention thousands of (night) hours of field testing.
Almost all have a nylon shell, but that doesn’t mean they’re all the same, even in that respect. Nylon can be thick or thin, sturdy or fragile, heat conductive or not. It can also be truly waterproof or merely offer a momentary delay in getting soaked. Look for strong, tight seams and composites that truly do the job.
The interior of sleeping bags has changed over the years, with more and more offering superior insulating materials. Some weigh as little as an ounce per square yard. Primaloft, Dupont Thermolite and other synthetics has made it possible to produce a lightweight bag that really keeps the cold out, while still allowing the interior to breathe.
Clever design has been added to ingenious materials in many models. Those that offer layered synthetic insulation with natural (or even synthetic) down provide excellent temperature control and great comfort. Fill is measured by a number, with 750 now the bottom for a good bag, 900 is better. The number represents the volume occupied by a single cubic ounce of material. Use it to compare bags.
Materials aren’t the only thing that’s important. Geometry has really been improved in contemporary designs. A full length zipper is important on those nights when it’s warm and you want to let a little air in. But having a well shaped mummy hood and draft collar are big advantages on those nights when you don’t.
The mummy hood helps keep your arm warm if you’re the type to put it under or above your head. It’s also designed to allow you stuff clothes and towels beneath a liner to make a pillow. At the other end, space has been expanded in some models to allow those who sleep on their backs to keep their toes pointing up in a comfortable position.
A bag should keep the cold ground out of the bag while allowing you to move freely. Those characteristics are hard to obtain together, but modern designers have accomplished just that.
At the same time, you want that bottom layer to provide sufficient padding when you don’t have an air mattress or cot. Some models accomplish that with a slide-in rollable pad, others have the padding built in. Look for ones that provide adequate comfort without adding excessive weight and bulk.
Your sleeping bag is the most important piece of large equipment you’ll take on a camping trip, unless you sleep in an RV or motel. There are those that would argue that isn’t really camping. A tent is important, but there are trips where you don’t want or need one. Your bag is your last line of defense, and your first line of comfort, for a great night’s sleep in the outdoors.
Get a great night’s sleep and you’ll be well-refreshed to tackle that hike the next day.