Heat stroke or heat illness is a form of hyperthermia that comes about as a result of exposure to external environmental heat rather than an internal lack of thermoregulation such as a fever. It is defined as a body temperature of 42 C (about 105 F) combined with an inability to regulate one's own body temperature. To avoid heat stroke, avoid alcohol and stimulants of any type in hot weather, wear very light loose fitting clothing, drink plenty of water, and cease exercising immediately if you feel light-headed or nauseous.