Abstract
Drowsiness and fatigue are serious problems in all transportation systems. One persistent issue is the lack of an agreed definition of these respective energetic states. This paper reviews the theoretical approaches (cognitive versus physiological) framing the driver fatigue problem. Known contributing factors to drowsiness include sleep debt, circadian rhythm, and shift work. However, it is also suggested that certain inherent physiological reactions engaged in responses to motion itself represent a previously unrecognized but significant source of fatigue. The impact of this factor is confirmed through comparisons of studies that either have or have not included prolonged motion.