A sleeping bag liner is a great way to protect and keep the inside of your sleeping bag fresh and clean, it's also an ideal way to add an extra layer to a thinner sleeping bag to keep you warmer in cooler climates. A sleeping bag liner is another great way to extend your bag's lifespan. It protects your bag from tears and other damage. It also helps keep it clean and will increase the temperature of your bag at night. Liners come in mainly cotton, silk or fleece. Which to choose? A 3-season sleeping and a silk liner. On cold nights use the bag and liner together. On warmer nights just use the sleeping bag, and on hot nights just use the liner.