Bod-i-Bag

This fleece sleeping bag liner looks like a really long hooded sweatshirt, except it has a drawstring base. You can tuck your feet in and close it up, but then wear it to get out of your bag at night to go pee or whatever. I got mine to combine with my Bivanorak bivvy bag to make a lightweight sleeping system, but it also does double duty as a garment that's very nice for sitting around and just keeping warm around camp. I've used it up in the mountains at about 8,500 feet with the temp down to about 38 F. It's light and packs up very small (mine is 9x15 and maybe 2 lbs), and is available with a stuff sack. Most importantly, they will custom make one for you if, say, you are very tall (I'm 6'10" and 260 lbs). You can also choose from a few fabric thicknesses and add a pocket pouch. I opted for the thickest weight fabric with the pocket pouch, which has a zippered mesh compartment. Great service, too. The maker got my special order to me in 4 days.