Objectives: We investigated the effects of fatigue on NK cell function and lymphocyte subpopulations in nurses performing shift work using a longitudinal design. Methods: Fifty-seven female nurses engaged in shift work at a hospital in Japan were selected for our study cohort. The hospital used a counterclockwise rotating three-shift system. Night shifts followed day shifts after a seven-hour interval. Immune parameters measured at the beginning of the day shift through to the end of the night shift were compared between two groups stratified by their level of fatigue. Statistical differences were evaluated after adjusting for baseline immune values and other demographic features. Results: Subjective feelings of fatigue increased progressively from the beginning of day shifts to the end of night shifts. From the beginning of day shifts to the end of night shifts, NK cell activity and CD16⁺CD56⁺ lymphocytes decreased, while CD3⁺ and CD4⁺ lymphocytes increased. The group with the greater increase in fatigue showed a larger decrease in NK cell activity and a larger increase in CD4⁺lymphocytes when compared with the group reporting less fatigue. These findings did not change after adjusting for demographic factors and sleep hours. Conclusion: Our data suggest that shift work has deleterious effects on NK cell function and that the effects depend on the degree of fatigue. Proper management of shift work may lessen fatigue in workers and also ameliorate many health problems experienced by shift workers.

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