The body usually maintains a constant temperature of 98.6 degrees Fahrenheit. At this temperature the cells of the body work best. If there is any significant change in temperature, it is sensed by an area of the brain called the hypothalamus. When the body gets too cold, this center alerts the rest of the body to begin warming up. Shivering, the rapid movement of the muscles to generate heat, then begins. Teeth chattering represents localized shivering.
From: Why do Men Have Nipples? by: Mark Leyner and Billy Goldberg, M.D.