A well-rested person is a happy person, and the same goes for being a happy camper. Falling asleep is rarely a problem in the backcountry, since you've been very active all day, but staying asleep can be another story. Combined with a good tent and pad, your sleeping bag is essential to peaceful slumber.
Like many gear selections, there is no one 'best' sleeping bag for you, unless you always camp in the same conditions, which sounds boring. So, be honest with yourself about what conditions you will mostly likely encounter, and then hedge your bet a little.
Down vs Synthetic
Heat transfer happens in three ways: radiation, conduction, and convection. Insulation works by creating a barrier that slows this process, either through density or dead air space. Sleeping bags use high efficiency lofted insulation to reduce heat loss as much as possible when you are resting and generating very little additional heat for yourself.
The main indicator of the value of insulation is how much loft it provides compared to compressibility and moisture tolerance. High quality down is the premium insulation based on warmth-to-weight ratio and compressibility, though it loses all insulation value when wet and must be thoroughly dried and re-lofted to be useful again. Synthetic insulation has water-repelling properties that allow it to continue to insulate when wet and dry relatively quickly, but is bulky and less compressible compared to down.
As covered above, getting a down sleeping bag wet can be a big problem, but really, a wet bag is hard to sleep in no matter what, so check the weather and be careful when transporting and setting up camp. But the question remains, if down if so hazardous when wet, why even take it into the woods? The short answer is that down compresses, carries, and insulates much better than synthetics, so much so that it is really the only choice for sub-zero rated bags. At ratings in the 20 to 30 range, the weight savings can be significant, and when you carry everything on your back, the ounces can add up. Also, because down is such tricky stuff to contain, the fabrics and construction on down bags will be better than synthetics.
For those of us who are 'no-matter-what' campers, however, the weight penalty of synthetics is offset by the ability to hang it in the sun after a wet night and be ready to go in a few hours. Synthetics are also less expensive as a rule, which makes investing in you kit that much easier.
Temperature rating is the most important descriptor of a sleeping bag. It is the manufacturer's determination of what temperature range the bag is designed to perform in. A good rule of thumb is to add 15 degrees to the stated rating of most big-brand sleeping bags. For example, in a tent on a sleeping pad, you will be warm in a 20 degree bag when the air outside is 35. Between 35 and 20 you won't be at risk, but not so comfortable. If you forego the pad or tent, expect to be cooler sooner. It's important to know that in the U.S., sleeping bag ratings are not standardized, so Your Mileage May Vary.
Construction and materials
When one ounce of down is prepared and tested in specific conditions, it will take up a certain volume, expressed as its Fill Power. 550 fill power lofts to 550 cubic inches, 800 takes up 800 cu in. The greater the fill power, the fewer ounces of down you need for suitable loft, but the price will go up for this higher quality material. Down sleeping bags are built with the inner and outer shells connected by lightweight mesh baffles that create chambers to hold the down and allow air movement but reduce insulation shift.
Conventional synthetic bags are made with a construction where horizontal batts of insulation are stitched to the inner and outer shells in an overlapping 'shingle' manner to increase loft and reduce cold spots. This can be bulky with low-end insulation but is a very reliable and time-tested design. Synthetic insulation will be described by the overall weight of the material used, and while many outdoor brand have their own insulation products, better bags will feature Polarguard insulation for maximum performance.
Newer synthetic sleeping bag construction takes advantage of fabric welding technology to stabilize a single high-loft insulation batt to the shells so that stitch holes are greatly reduced. This is significant because heat is lost through the stitch holes, and the bulk and weight of the seams and thread are eliminated. These bags are highly compressible and light weight, but do no feel as 'lofty' as their conventional cousins.
So, what type and temperature bag should you get? This can be a bit tricky, because the 'best' sleeping bag for August in northern New Mexico is not the best for the same day in Ithaca or Springer Mountain, but you can probably hedge your bet. You should already know whether you sleep 'warm' or 'cool,' what kind of conditions you prefer, and the other gear you use on the trail.
The majority of sleeping bags for backpacking will be made of thin nylon fabric and have some version of a mummy shape, with a much narrower foot than shoulder, and frequently a hood. This shape is intended to reduce interior volume that must be warmed and cover your head, a major source of heat loss on the body. With a more efficient design, the bag needs less material and insulation to keep you warm. The weight and space savings will increase with the price, while durability may decrease.
As tempting as having a bit more room may sound, you will lose the efficiency of the low interior volume, and folding the extra under or filling the space with clothes is not really a solution. Getting the right size bag means you are carrying only as much gear as you need. A 'Regular' sized sleeping bag is designed for a 5ft 11in male of about 175lbs. The 'Long' size will accommodate up to about 6ft 6in, the 'Short' someone in the 5ft 6in range. While women certainly can go for a Short bag, they may be happier in a womens-specific model. With a modified shape and additional insulation in the hips and feet, a Womens Regular fits someone to 5ft 5in, Long to 5ft 10in.
Where weight and space are less of a consideration, you will see more rectangular shapes and heavier materials better suited for car or family camping, hunting, or sleepovers.
Key features to look for:
Full-length two-way zippers will allow you to vent your feet at night and/or open the bag up more fully to air out or in warmer conditions. Half-zip styles will save weight and bulk, but limit the temperature range.
A shoulder baffle forms a seal that can improve the temperature rating of a bag but adds very little weight.
Expedition pocket on the outside means your alarm watch won't be muffled by the insulation down by your waist.
Fleece lined foot can be nice for cold toes, but adds weight and bulk. frequently found on Women-specific bags.
Checking it out
When seriously shopping for a good sleeping bag, be prepared to try them on. In the store make sure you take your shoes off and empty your pockets, then find a pad very much like what you will be using. Spread out the bag and take a good look at the whole thing, head to foot. Climb in carefully and remember that the ground will be a bit more forgiving than the slab in the store. Zip up completely, roll around, get back out. The more you try out, the clearer your preference will be. Remember - one third (or more!) of every day in the backcountry will be spent in there.
Making it last
All sleeping bags are machine-washable, in the biggest front-loading machine you can find and using a laundry soap with no whiteners, brightener, bleaches, softeners, or fragrances. We suggest Tech Wash or Down Wash from NikWax. A short trip through an equally large dryer at low heat will restore the loft to the insulation. For down bags, three new tennis balls in a clean sock will make a big difference.
When not in use, make sure your bag is not compressed and is aired out. If you have a very tall closet, it will benefit from being hung up, or turn it inside out and stuff very loosely in a mesh or cotton storage bag.