This is because humans are homeothermic animals i.e. having a body temperature that is constant and largely independent of the temperature of its surroundings. In a cold environment, body heat is conserved first by constriction of blood vessels near the body surface and later by waves of muscle contractions, or shivering, which serve to increase metabolism. Shivering can result in a maximum fivefold increase in metabolism. Below about 40°F (4°C) a naked person cannot sufficiently increase the metabolic rate to replace heat lost to the environment.