The THERMOLITE Reactor Extreme liner is Sea to Summit's warmest knitted sleeping bag liner. If your adventures take you to colder climates, the Reactor Extreme will add plenty of warmth to a sleeping bag. In warmer weather, it can be used a 'stand alone' sleeping bag. The hollow core fibers of the THERMOLITE fabric work like hollow polar bear or caribou fur - along with the knitted structure, this traps plenty of insulating air. Super soft and amazingly stretchy, the Reactor Extreme uses a heavier version of THERMOLITE to provide extra warmth for an incredibly comfortable night's sleep. Use one to build your cooler-weather sleep system and to help to keep your sleeping bag clean, too.