What Our Gearhead® Experts Are Saying:"I started using the Sea to Summit Reactor Mummy Drawcord Sleeping Bag Liner on shoulder-season trips where my bag was borderline for the temperatures, and it closed that gap consistently without adding much bulk to my kit. The drawcord footbox is the detail that sold me on this version specifically, since being able to open the base and walk around camp without fully getting out of the liner is genuinely useful on cold nights."A good sleeping bag liner does more than add a degree or two of warmth. It extends the range of your existing sleep system, keeps your bag cleaner over long trips, and makes the whole experience more comfortable without asking much in return. The Sea to Summit Reactor Mummy Drawcord Sleeping Bag Liner delivers on all of that and adds a few smart details that make it worth reaching for on every outing. Thermolite Pro fabric uses hollow-core fibers woven with infrared-absorbing ceramic pigments to reflect your body heat back toward you, delivering a meaningful thermal boost in cool and cold conditions without the weight of additional insulation. The stretch-knit construction moves freely with your body through the night and works with sleeping bags of any shape, so you are not locked into a specific setup. The drawcord footbox is the standout feature of this version, letting you open the base to walk around camp while still wearing the liner, which anyone who has made a cold midnight bathroom trip knows is worth its weight in sleep. Shoulder openings allow arm movement without fully escaping, and HeiQ Fresh odor control keeps the liner comfortable across extended use between washes.