Mummy bags have a tapered shape. They don't give you as much room to move around, but they conserve body heat well, and they pack up a lot lighter than rectangular sleeping bags. Three-season mummy bags with synthetic fill are a good mid-range choice -- not as expensive as down, and not as heavy as a year-round sleeping bag. Most three-season mummy bags are good down to about 20 degrees F.
The North Face Cat's Meow (Est. $170 and up) gets more reviews than any other three-season sleeping bag with synthetic fill. This 20-degree bag won its first Editor's Choice award from Backpacker Magazine back in 1993, and earned the Editor's Choice Gold Award in 2006 when it used Polarguard Delta fill. The current insulation is Climashield Prism, which is even lighter, softer and more durable – and holds more warmth when wet. The Cat's Meow has high-end conveniences (like one-handed hood adjustment and zipper pulls that glow in the dark) and comes in left- and right-zipper versions so you can zip two together to make a double sleeping bag.
The less expensive The North Face Blaze is also rated to 20 degrees, but at 3 lbs. 2 oz., this mummy bag weighs six ounces more than the Cat's Meow. The zipper extends around the footbox so you ventilate your feet on warmer nights, or even open the bag flat to use as a quilt. This is one of the top-rated sleeping bags at retailer site Moosejaw.com. Owners praise the sleeve at the foot where you slip in a footwarmer, and as on the Cat's Meow mummy bag, there's a similar pocket for a handwarmer.
The Sierra Designs Verde 20 is even heavier at 3 lbs. 5 oz., and lacks footbox ventilation. However, the Verde 20 wins the Gear of the Year Award at Outside Magazine, where editors praise this 20-degree bag not only for good performance, but for using Climashield Green. This synthetic fill has the same excellent properties as the Climashield insulation used in The North Face bags, but is made with at least 40 percent recycled soda-pop bottles. In fact, the only parts of the sleeping bag that don't use recycled materials are the zippers and Velcro. The smaller women's version, the Sierra Designs Déjà Vu 20 weighs less at just 2 lbs. 13 oz.
If you move around a lot in your sleep, you'll appreciate the well-recommended MontBell Super Stretch Burrow sleeping bags. Elastic baffles stretch up to six inches as you move, but keep the down in place so you stay warm even when sleeping on your side. Two-way zippers let you ventilate any part of the body that's too warm. The proprietary ExcelSoft insulation is similar to Primaloft Sport, absorbing less than three percent of its weight in water. The three-season MontBell Super Stretch Burrow #1 is rated for 15 degrees and weighs 3 lbs. 14 oz. The weight is really the main drawback.
The Mountain Hardwear Ultralamina series is recommended at Backpacker Magazine for its welded seams, which "reduce bulk and improve moisture resistance." As a three-season mummy bag, editors recommend the Ultralamina 15, which weighs 2 lbs. 13 oz. It has partial-length zippers on both sides – handy for sitting up in bed – but quite a few owners say they wish the zippers were full-length for better ventilation. At Moosejaw.com, owners like the less expensive Mountain Hardwear Lamina 20 – two ounces heavier but with full-length zippers.
If $150 or more strains your budget, Backpacker editors and other reviews recommend the 20-degree REI Polar Pod +20 (*est. $80), but it weighs 3.5 pounds and may be hard to find. The Thermolite insulation is also considered a budget synthetic fill, without as much loft and therefore less warmth per ounce. An in-depth review at BackpackGearTest.org finds it comfortable down to about 25 degrees.