What Our Gearhead(R) Experts Are Saying: "This liner adds a surprising amount of warmth without feeling restrictive. The stretch-knit Thermolite fabric moves with me, and the shoulder openings make late-night adjustments super easy. Whether I'm pushing a lighter bag into colder temps or just wanting a cozier sleep system, the Reactor has become a go-to." We reach for the Sea to Summit Reactor Lightweight Liner when chilly nights call for a little extra insulation in our sleep system. Built with Thermolite Pro's hollow-core fibers and infrared-reflective ceramic pigments, it delivers an impressive thermal boost by capturing and returning our body heat. The stretch-knit construction offers freedom of movement and allows the liner to conform naturally to the shape of any sleeping bag. Designed for versatility, the Reactor comes in multiple mummy variants. Shoulder openings on the classic mummy let us reach out without fully exposing ourselves to the cold, while the drawcord footbox on select versions provides ventilation or mobility around camp. HeiQ Fresh odor control helps keep things pleasant on extended trips, and the compact packed size makes it easy to stash for any cool-weather mission.