Sea to Summit Spark Ultralight Sleeping Bag 40

The Sea to Summit Spark Ultralight Sleeping Bag 40 is a lighter, summer-oriented member of the Spark line, rated to roughly 9°C comfort and a 4°C (40°F) limit — built for warm, frost-free nights rather than shoulder-season trips with temperatures dipping toward zero. Its 180g of 850 FP, RDS-certified goose down carries the hydrophobic, PFC-free Ultra-Dry Down treatment and is arranged in simpler sewn-through baffles — a lighter and cheaper construction than box baffles, though somewhat more prone to minor cold spots along the seams. Shell and liner are both 10D DWR-treated nylon, and the bag uses a half-length left-side zipper with no two-way opening, plus a draft tube running alongside it. At 340g total weight and compressing into a 1.5L stuff sack, it's among the lightest options for summer ultralight backpacking, where every gram matters. The unisex cut and a recommended max user height of 182cm mean it suits smaller-to-average frames better than taller users.