Shivering is the body’s natural way of keeping warm and can actually serve as a life saver in extreme cold. Shivering produces heat by forcing skeletal muscles to contract and relax rapidly. Heat is produced as a byproduct when muscles metabolize ATP (adenosine triphosphate) for contractions. Approximately 80 percent of the muscle energy used in this process is turned into body heat. One study has shown that warmth from external sources such as blankets and hot water bottles can actually be harmful in some cases of hypothermia because the shivering reflex is shut down.