The bottom of a sleeping bag typically does not provide significant insulation, because body weight crushes the loft of the insulation material. Due to this, it is necessary to use a pad or other less crush-able insulation underneath the sleeper, especially in cold weather. Due to this, some sleeping bags do not include insulation on the bottom. Some include a sleeve for holding a sleeping pad. Additionally, some campers, especially ultralight backpackers or hammock campers, have started to use a top quilt, essentially a sleeping bag without a back. Some top quilts include a foot box, while others are just simple blankets.
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