The Sea to Summit Reactor Midweight Fleece Liner is a versatile thermal boost designed for every adventure. Made from stretch-knit fleece with integrated armholes, it provides exceptional freedom of movement—perfect for side sleepers or restless tossers. It fits inside any sleeping bag and snaps into Sea to Summit bags for easy entry and exit. Powered by Thermolite® Pro with Far Infrared (FIR) technology and hollow-core fibres, the Reactor Midweight efficiently traps and reflects body heat, adding up to 6 °C 11 °F of warmth without extra bulk when used as part of a sleep system. Unlike a full sleeping bag, the liner is quick-drying and easy to clean, with HeiQ Fresh odour control to keep it smelling fresh on extended trips. Using the liner also reduces how often you need to wash your sleeping bag, helping maintain loft and extend its lifespan. Features Freedom to Move: Stretch-knit fabric and armholes allow unrestricted movement in any sleep position Thermal Boost: Thermolite® Pro with FIR and hollow-core fibres traps and reflects heat, adding up to 6 °C 11 °F Easy to Clean: Quick-drying with HeiQ Fresh odour control for multi-day trips Bag Compatibility: Fits any sleeping bag and snaps into Sea to Summit bags for easy entry/exit Extend Bag Lifespan: Reduces frequent washing, maintaining insulation loft over time Adventure-Ready: Ideal for backpacking, hut trips, bikepacking, and travel Sizing & Weight Short (fits up to 170 cm): 330 g Regular (fits up to 185 cm): 350 g Long (fits up to 198 cm): 355 g