you tuck in for the night and place it in any colder areas of your sleeping bag. if you do not have a water bottle to use, or hot water, you can always use hand warmers, or heater packs, both of which are disposable. you can place these in your sleeping bag as well, or use them in your hands.
some old school campers would heat rocks at the campfire and put them in a sock and then put them in the sleeping bag. as long as the rocks are not too hot, you can feel free to try this method out. see our article on the best water bottle for 