The THERMOLITE Reactor Fleece sleeping bag liner is a luxuriously soft and super warm liner that can bump up the warmth of your sleeping bag considerably. It can also be used as a stand-alone sleeping bag in warmer climates and has a quarter length zipper to control temperature and allows the user to get in and out of easily. The hollow core fibers that make up this fabric trap heat effectively, keeping you toasty warm. If you're looking for a high warmth to weight ratio or a lightweight summer bag, this is an ideal choice.