.....
Also can take rocks, wrap in foil, put by fire then put in the bag. Smaller river rocks work well.
You gotta watch out about putting wet rocks in a fire. Sometimes they explode....steam is a powerful force.
I've put hot water in the nalgeen, but it still gets cold before long. I've pissed in a bottle too, but then I don't want to use the bottle anymore...
. I had good results with the therma patches from walgreens. Peel em and stick em to your t-shirt. They get pretty hot, so better to keep something between your skin and the patch. They last a while....don't take up much room in a pack.
The main thing is to get your body off the ground and keep your head warm. I have a therma rest air matress for backpacking and a walmart air matress for car camping....I kind of hope it's gonna be cold.