We added a new temperature rating to the 2015 Backpacker Editors' Choice Award Winning Sonic™ series of sleeping bags. Built to put a smile on your face when the cold weather is keeping most people out of the mountains. We've used all the best ingredients of high-end fabrics and 850 fill power down insulation paired with stretch construction at the knees for side sleeping. Thermo Gills™ regulate the internal temperature of the bag by as much as 20° F without letting drafts in.