I suppose that you use your Nalgene bottle in the bottom of your sleeping bag to keep your toes warm. I don't put mine inside my sleeping bag. Too worried about it maybe leaking.
KC
I typically use the opaque nalgene with the blue top, they are the lightest cheapest ones and have never had a leak in my bag. I don't think I would trust anything but a time tested nalgene for this application.
Screw the lid on tight and give it a shot sometime. It takes a bit of faith but makes for a warm bag. Especially helpful for stretching a summer sleeping bag into the shoulder seasons.
