Sea to Summit Camp & Hike Spark Spiii Sleeping Bag Regular S2131. Each model uses premium materials and no-frills design to provide cutting-edge performance for the designed end-use. Ultralight does not mean ultra-tight: The Spark mummy sleeping bags are snug but still have adequate girth for the performance-oriented backcountry user. IS THIS DOWN MUMMY SLEEPING BAG…? Suitable for cooler temperatures? Yes. You would want the Spark 5 F or possibly the 18F – either bag will work in late Fall the 5F will take you down to winter temperatures when paired with an adequately insulating sleeping mat like the Comfort Light Insulated. Really suitable as a sleeping bag liner? Yes. You could use the Spark 50F or even the Spark 40F or 28F to boost the performance of a bag for cold-weather use. Adding one of these to a 20F / -5C bag might make more sense than buying a 0F / -20C bag for only occasional winter use. Able to be zipped to another sleeping bag? No. Even though the 18F or 5F have full-length side zippers they are not designed to be coupled to a sleeping bag with a right zipper (for instance the Flame 18F or 5F).