The Sea to Summit Spark III is the lightest and most compact sleeping bag in our review. It does give up some luxury and versatility in order to achieve its amazingly low weight and minimal size; it sports the tightest dimensions among bags we tested and only a 1/3 length #3 YKK zipper that was both the shortest and the smallest zipper of any bag we in our review. What the Spark III doesn't give up is warmth; in our real world testing, it proved to be a real-world 25F sleeping bag and was even warmer than a few other 20F bags we tested. While we loved this bag for how light and small it is, it's worth noting for folks worried about its small dimensions, that the Western Mountaineering MegaLite, our Editors' Choice award winner, and the Marmot Phase 20, are only a couple ounces heavier. They are also nearly as packable (but not quite as packable), yet offer larger interior dimensions. The Spark III previously won our Top Pick for the best lightweight model and remains a top contender but the Marmot Phase 30 is our new Top Pick because it is even lighter at 1 lbs 2 oz and packs a little smaller. Though it's worth noting that while we thought the Phase 30 was warm enough for most backpackers, if was more like a 35-40F bag, so if sleep a little on the cold side or find yourself frequency sleeping at higher elevations or colder locations the Spark is a LOT warmer for only a tiny bit more weight and packed volume.
The Spark I looks the same as the Spark III, but it's designed for warm weather camping. Retailing at $300, it's way less expensive than its beefier sibling that we reviewed. The Spark I's temp rating is 46F, and Sea to Summit claims it only weighs 12.3 oz and compresses to a tiny 1.55 L. If you frequently overheat in your bag, or camp frequently in warm nighttime temps, the Spark I saves you a lot of weight and cash.