The Sea to Summit Spark SpII Sleeping Bag is ultralight for backpackers, racers, bicyclers, and alpine climbers. Primarily focusing on giving you the lightest bag out there, Sea to Summit stuffed the Spark with 850 fill down for the best possible warmth to weight ratio. This bag's Pertex Quantum shell is windproof and prevents the down from breaking through, and the smooth nylon lining adds some durability. Going beyond an ultralight design, Sea to Summit added its Ultra Dry Down treatment to help this bag resist moisture and enhance your comfort regardless of the conditions. The Spark SpII has a contoured mummy shape to closely fit your body while you sleep. Its full hood wraps around your head to eliminate the possibility of waking up with freezing ears, and the 13 length zipper on the left helps maximize warmth. The Spark compresses so small that Sea to Summit had to re engineer a new compression sack for it, ending up with a sack that can fit in the palm of your hand. So when you get this bag, you might be able to stuff your pack with a few extras even if you are a minimalist.