Sleeping outdoors in a natural shelter and surviving below zero temperatures

I ventured into the wild as winter set in and the temperatures plummeted dramatically. I made the decision to spend the night in a natural shelter deep in the woods. I attributed the strange noises I heard at night to animals doing what animals do. It's generally perfect to get out among the trees to rehearse my bushcraft and I had a very much required night by the fire under the careful focus of an old oak