What Our Gearhead(R) Experts Are Saying: "The Spark 45F is my go-to when I want to keep my pack as light as possible without sacrificing real sleep. The down lofts quickly, the hood seals warmth better than most ultralight bags, and the 10D shell breathes incredibly well. It's perfect for warm trips or pairing with a liner to extend the season." We turn to the Sea to Summit Spark 45F when our goal is to move fast and carry almost nothing without compromising comfort. This ultralight bag is packed with 850+ fill Ultra-Dry Down that resists moisture while offering impressive loft, delivering reliable warmth for warm-weather adventures and minimalist summer objectives. The breathable 10D liner feels soft against the skin, while the 10D DWR-treated shell and water-repellent hood and footbox shed condensation and breeze throughout the night. A technical hood and draft tube help retain heat efficiently, and the half-length zipper keeps weight low without limiting ventilation when nighttime temps stay mild. For ultralight backpackers counting grams, the Spark compresses down to a shockingly small size while still providing the comfort needed for restorative sleep. Whether used alone in warm conditions or paired with a liner to stretch the season, it's a remarkably capable bag that thrives on long miles and light packs.