The analysis of human alertness rates shows that hypo alertness episodes occur at foreseeable periods in the daytime. The alertness drop of the human operator is one of the main causes of incidents and accidents in road and rail transport. The peak of accidents risk through hypo alertness has been detected in the middle of the afternoon, late at night and early in the morning, which complies with the circadian patterns of drowsiness. As far as we know, in the field of rail transport safety, the chronobiological aspect of alertness has not yet been addressed. However, the rotation rate of the human operators work schedules sets the same problem as shift work. To take into account the chronobiological data can improve safety, proposing solutions to adapt work schedules to the circadian changes of the human operator functional capacities. To determine the times of best performance by the human operator and to make them match his work schedules improves the reliability of the man-machine system and reduces the risk of accident.