High altitude is an environment that is alien to man. Stressors associated with the mechanics of flight include motion and acceleration forces applied in three vectors and in degrees that are foreign to the human vestibular apparatus. Furthermore, the work patterns of a pilot can interfere with lifestyle and circadian rhythms. Therefore medical fitness is an important consideration in determining an individual's suitability to exercise a pilot's licence. The medical standards applied depend on the type of aircraft flown and the duties expected of a pilot. There are three broad categories of pilot. In ascending order of stringency of medical standards these are the private pilot, the professional pilot and the military pilot.