About 20 years ago, I was hunting late season (December back then) and there was about a foot of snow on. It was one of those days when the weather people kept saying "unless you MUST go outside, then DON'T". Well, it WAS Saturday in Bow Season, so I MUST go! LOL. The wind chill was in the negative 20's and it WAS dangerously cold. I was ALSO hunting alone (in my 20's not real smart then, not much better now) and about a mile and a 1/4 from the truck. I was the only guy hunting Schell Osage CA that day in WCentral MO. I started thinking about keeping warm that night before and devised a plan. No one I knew had ever tried it, and I thought I was probably the only one to do it still on occasion. I am glad to hear there are others! I carried a sleeping bag in with me, hunting out of a portable homemade stand with a VERY small platform. I DID have one idea that I haven't seen presented here that I wanted to share with whoever. I bought a pair of Camo suspenders at Walmart and they have BIG alligator teeth clamps on them. I extended them as far out as they would go, and clamped them on the top of the sleeping bag opening. I had snow on my boots and water from walking in, but I just went ahead and climbed into the bag and sat down after getting EVERYTHING ELSE set up in top of the tree. I pulled it up until my boots were in the bottom of the bag and spread out my legs so that each foot was in opposite corners of the bag. Then I pulled it up to my armpit/chest area and stuck my arms and all down inside. In spite of the extreme weather conditions, I have NEVER been any warmer on the stand. Of course I had my head covered up good and my down coat on, but I'm telling you, THIS WORKS! I have done it several times since then.
Here's the cool part! The suspenders play an important role. If you extend them all the way out, they reach below your waist a little bit. When I saw deer (yes I killed one that weekend, and have killed about 3 or 4 out of a stand in a sleeping bag since then!) I just stood up as usual and allowed the bag to "drop" slowly down my torso until the suspenders caught it, freeing my entire upper body, but not allowing the bag to get around my feet. This is NOT the safest I have ever been in the stand, and you BETTER be careful or it could be your reason for a fall, but if you do it right, and don't do foolish things while in the bag, you CAN stay warm on the WORST days and be fairly safe doing it.
Glad to see others have done this as well. Although they make the "heater body suit" which is basically a sleeping bag idea but much more expensive, you can do this with what you already own usually and a $8 pair of suspenders. I used a bag that had a nylon outer shell the first time, I have since bought a camo sleeping bag with a cloth outer shell.
Hope this idea helps someone.
Bowhunter: The ORIGINAL "Tree Hugger"