Sea to Summit Reactor Fleeceweight Sleeping Bag Liner

The Sea to Summit Reactor Fleeceweight Sleeping Bag Liner is a 365 g mummy-shaped liner that combines Thermolite hollow-core fibres with grid fleece, adding up to 8°C/14°F of warmth when used inside an existing sleeping bag. That's a meaningful boost for pushing a 3-season bag into colder conditions than it's normally rated for, useful on an unexpectedly cold night or as an alternative to carrying a heavier bag. The fleece lining also makes it substantial enough to double as a standalone sleeping bag in warm weather, so one piece of kit can cover both a cold-weather liner role and a hot-summer bag role. At 365 g it's noticeably heavier than a plain synthetic liner, and recycled materials plus bluesign certification are part of the fabric, relevant if sourcing matters to your gear choices; anyone chasing minimum pack weight should weigh that 365 g against how much extra warmth they actually need.