What Our Gearhead® Experts Are Saying:"I brought the Reactor Fleece Mummy + Drawcord Sleeping Bag Liner on a late October trip where temps dipped further than expected, and it genuinely saved me from a miserable night. The shoulder zippers made it easy to vent when I overheated on the climb in, and the drawcord footbox locked in warmth the moment temps dropped after midnight."The forecast said low thirties, and your sleeping bag says it's rated to twenty, so on paper, you're fine. But anyone who has ever woken up stiff and shivering at two in the morning knows that paper and reality do not always agree. The Sea to Summit Reactor Fleece Mummy + Drawcord Sleeping Bag Liner exists for exactly that gap, slipping inside your existing setup to add a meaningful layer of warmth without adding meaningful weight to your pack. Sea to Summit built the Reactor Fleece liner around hollow-core Thermolite Pro fibres arranged in a grid-pattern fleece that traps heat more efficiently than a standard flat fabric would. Shoulder zippers give you a quick escape valve when you're too warm, and the drawcord footbox lets you cinch down or open up depending on conditions. HeiQ Fresh odour control keeps things tolerable on longer trips, and the mummy shape means warmth stays where your body needs it most. It also integrates directly with Sea to Summit's Free-Flow Zip sleeping bags, making it a natural pairing if you're already in that ecosystem.