There is an interesting discussion about how best to get warm/sleep warm in your sleeping bag over on the backpackinglight discussion boards. While the original post was about an REI blog that says: “When you wear extra clothes to bed when camping, you create more barriers between your body heat and the insulation of your sleeping bag. If those layers aren’t very permeable (like a waterproof jacket or puffer coat), then your sleeping bag won’t warm up properly and you’ll ultimately be relying on the insulating properties of your clothes instead.”
The poster then goes on to say; "I think they are wrong. Any heat that gets past your clothing will be trapped inside the sleeping bag. There is nothing magic about a sleeping bag, they are just an insulator layer after all. It isn’t the sleeping bag that needs to be warmed up, it’s your body that needs to be warmed-up."
"Your thoughts on this?"
Well a couple of things to digest here. First of all, we can't find that post anywhere on the REI website....so maybe it got pulled down, or maybe the poster is incorrect about finding it there. Or maybe we don't know how to search REI's website. But the topic is an important one, and the discussion that follows this post is excellent:
You may need to join to follow the whole thing. But one of the follow-up posts cites a fascinating study: https://www.sitkagear.com/experience/a-navy-seal-rewarming-drill
And that seems to be a more appropriate response. Getting naked doesn't get you warmer. But the human body does generate a ton of heat, if you can insulate it enough for it to do the job., If you get hypothermic, get insulated and stay insulated.