When your body begins to lose heat faster than it can produce it, you may voluntarily exercise to keep warm. This tends to drain away your energy reserves. When they have been depleted, your internal body temperature begins to decrease, cooling your vital organs. At this stage you are becoming hypothermic. Without treatment this can lead to mental confusion, stupor, collapse and death.
Prevention is the best cure.
Avoid exposure, stay dry and beware of the wind. Change damp undergarments. Eat high fat and carbohydrate foods. Camp before exhaustion sets in. Useful sources are pre-warmed garments and a dry sleeping bag, a warm tent interior, body heat from companions and warm liquids.