This study examined the stress states experienced by nuclear power operators at work under an existing shift system, time-on-shift effects and the manner in which these states may be moderated by achievement motivation and a sense of coherence. The results show an incidence of stress states during evening shifts and night-shifts in the operators, which are primarily characterized by increased distraction as well as by enhanced sleepiness and low irritability. The night-shift was found to be the most problematic one in terms of increased sleepiness and distractibility, and reduced alertness. An abrupt fall in alertness during the evening shift and a considerably increased distractibility at its end was observed. These facts, as well as the lack of recuperation from daily domestic activities and caring for children at the start of the shift, suggest that besides the night duty, the evening one also constitutes a serious challenge to operational safety at this nuclear facility. Correlations found between personality characteristics and sleepiness, distractibility and irritability, indicate personal resources as important modifiers of stress states. A narrowing in the effects of motivation and sense of coherence was found in the evening shifts and night-shifts. The results have practical implications for intentional modifications of personal resources at nuclear facilities.