It has now become fairly routine for the human performance investigator to dig deeply into individual "life-style" issues in order to learn what may have affected the performance of a pilot, engineer, ship's captain, or truck driver. Virtually always, an attempt is made to reconstruct the on-duty/off-duty/rest/sleep/wake history of the key operational personnel involved in an accident. Frequently, data are ambiguous, so the true incidence of fatigue as a causal or contributory factor is largely unknown. (Author abstract)