1. A method for detecting driver fatigue in a vehicle including a memory device in which a predetermined fatigue model is stored, a position detector configured to detect a geographical location of the vehicle on a map, a sensor configured to detect at least one activity of a driver, and a sensor located in a cluster instrument, configured to detect ambient brightness comprising: analyzing activity information in the fatigue model, wherein the fatigue model is independent of a biorhythm model; determining a relative position of the sun based on a latitudinal coordinate of the detected geographical location; and weighting the predetermined model in accordance with the detected ambient brightness and in accordance with a known brightness based on the determined relative position of the sun.