What Our Gearhead(R) Experts Are Saying:"I use this liner when I want extra warmth without adding bulk to my pack. The stretch fabric is genuinely comfortable, and the armholes are super practical for late-night camp tasks. It's also much easier to wash than a full bag, which keeps my sleep system fresher on long trips."We bring along the Reactor Midweight Liner when our nights wander into shoulder-season cold or when we want cleaner, more comfortable sleep on extended trips. Sea to Summit builds the liner with stretchy knit Thermolite Pro fabric, giving us warmth, mobility, and a soft hand that works inside any sleeping bag. Available in Short, Regular, and Long sizes, it fits a wide range of heights without restricting movement. Far Infrared Technology helps trap and reflect body heat, adding noticeable warmth-up to 6degC or 11degF-on colder nights. Snap-closure arm openings create a practical, free-flow design that lets us sit up, cook, read, or prep gear without fully exiting the bag. On multi-night trips, the liner helps extend the lifespan of our sleeping bag by reducing the need for frequent washing, and it dries quickly after rinsing.