The Sea to Summit THERMOLITE Reactor Fleece Liner is Sea to Summit's warmest sleeping bag liner. Add one to your sleeping bag before heading into really cold environments or use it as a 'standalone' sleeping bag when the weather is warmer. The core THERMOLITE. fibers have the same hollow structure as a polar bear's fur, which traps a high level of insulating air. Luxuriously soft, the Reactor Fleece is perfect as part of a cold weather sleep system - plus it will help keep your sleeping bag clean.