Add an extra season to your sleeping bag or tweak the warmth factor with a Sea to Summit "Reactor" sleeping bag liner. Fleece or Thermoliteï¸' with long version. Use your Reactor sleeping bag liner to increase the warmth of your mummy sleeping bag or use it as a stand alone sleeping bag! These super cozy sleeping bag liners come in several options including regular, "Extreme" and fleece with long versions. Each liner has a tapered shape with a drawstring hood and quarter zip so they are warm and easy to use and warm. They are also small and light and come with a stuff sack. Sleeping bag liners are hygienic because they're easier to wash than your average sleeping bag. They're incredibly versatile since you can use them to tweak the temperature rating of your favorite sleeping bag for cold.