The ability to manage human fatigue in transport operations would be improved by a shared understanding of what fatigue is, how it should be measured, and how it affects safety performance. This may be achieved by a broader operationalization of fatigue, which would allow commonly studied aspects of fatigue to be considered alongside each other, and make explicit those aspects of fatigue that individual studies do and do not account for. According to such an operationalization, fatigue should be measured in terms of experience, physiological state and performance. In studying the effects of fatigue on operators, there would be a need to for greater consideration of its longer-term effects, and its motivational aspects. To understand its performance effects would require that we attend to the systematic interaction of sleep history, time of day, time at work, and time on task, in the context of factors describing various aspects of work and non-work life.