Sea To Summit Spark II

The Sea to Summit Spark II is a mummy-shaped down sleeping bag rated to EN/ISO 23537, with a -2 °C lower limit, 4 °C comfort rating and -18 °C extreme rating — figures that place it as a three-season bag for spring through autumn rather than winter use. At 490 g total with 300 g of 850+ fill power hydrophobic goose down (90/10 ratio), it is built for hikers counting grams on multi-day trips. The half-length zipper and box-baffle construction are the main weight-saving levers, giving a warmth-to-weight ratio around 4.1 and roughly 245 g per degree of comfort. The trade-off is no neck baffle and reduced ventilation versus a full-length zip, so cold shoulders and cool necks are more likely on frosty nights unless paired with a hat. The differential fill distribution (roughly 55% torso, 45% legs) concentrates warmth where it matters most while sleeping. With a 10D nylon shell, DWR finish and fitted hood, it suits thru-hikers and UL backpackers prioritizing pack weight over bells and whistles, provided they can manage the ventilation trade-offs themselves.