Why does work cause fatigue? A real-time investigation of fatigue, and determinants of fatigue in nurses working 12-hour shifts.

Background: One of the striking regularities of human behavior is that a prolonged physical, cognitive, or emotional activity leads to feelings of fatigue. Fatigue could be due to (1) depletion of a finite resource of physical and/or psychological energy or (2) changes in motivation, attention, and goal-directed effort (e.g. motivational control theory).

Purpose: To contrast predictions from these two views in a real-time study of subjective fatigue in nurses while working.

Methods: One hundred nurses provided 1,453 assessments over two 12-hr shifts. Nurses rated fatigue, demand, control, and reward every 90 min. Physical energy expenditure was measured objectively using Actiheart. Hypotheses were tested using multilevel models to predict fatigue from (a) the accumulated values of physical energy expended, demand, control, and reward over the shift and (b) from distributed lag models of the same variables over the previous 90 min.

Results: Virtually all participants showed increasing fatigue over the work period. This increase was slightly greater when working overnight. Fatigue was not dependent on physical energy expended nor perceived work demands. However, it was related to perceived control over work and perceived reward associated with work.

Conclusions: Findings provide little support for a resource depletion model; however, the finding that control and reward both predicted fatigue is consistent with a motivational account of fatigue.