Sea to Summit Reactor Midweight Sleeping Bag Liner

The Sea to Summit Reactor Midweight Sleeping Bag Liner is a 330 g mummy-cut liner built from Thermolite Pro fibres, rated to add up to 6°C/11°F of warmth when used inside an existing sleeping bag. The stretch-knit fabric moves with the body rather than restricting it, and snap-on shoulder connectors keep the liner from bunching or twisting overnight. This is a warmth-boosting layer for a bag that's a little too cold for the conditions, not a standalone sleeping bag - useful for stretching a 3-season bag into cooler shoulder-season nights, or as backup warmth if temperatures drop below forecast. At 330 g it's heavier than a pure summer liner, which is the trade-off for that extra insulation.