The name sleeping bag liner gives an unfair impression of these handy and versatile items. Far from being merely an appendage, useful only in combination with their bulkier siblings, these liners can replace sleeping bags in warmer climates, while taking up only a fraction of the space in your luggage. Even in colder destinations, they can transform a dingy duvet or blanket into a pristine cocoon. Of course, when used with a sleeping bag, they keep it clean and make it one or two degrees warmer.