Chronotype in relation to shift work: A cohort study among 37,731 female nurses

Chronotype may affect tolerance for circadian disruption induced by shift work. This study examines the association between chronotype, self-reported sleep timing, shift type preference, and sleep problems among nurses, and studies chronotype stability over time. The study included 37,731 Dutch female nurses who completed a baseline (2011) and follow-up questionnaire (2017), with information on shift work (e.g., job history, shift type preference [collected in 2017 only]), and sleep characteristics (e.g., chronotype, preferred sleep-wake time in a work-free period [collected in 2017 only], and sleep problems between working days according to Medical Outcomes Study-Sleep Problem Index II [MOS-SPI-II]). The association between chronotype and sleep timing was examined using (age-adjusted) linear regression. Associations between chronotype and shift type preference and sleep problems (MOS-SPI-II >30) were examined using ordered logistic and Poisson regression, respectively. With later chronotype, midsleep time increased (definite evening vs. intermediate types [reference]: β = 55 min, 95% confidence interval [95% CI]: 54-55), the odds ratio (OR) for 1-point increase in preference for night (2.68; 95% CI: 2.48-2.90) and evening shifts increased (OR 2.20; 95% CI: 2.03-2.38), while the odds for day (OR 0.17; 95% CI: 0.16-0.18) and morning shifts (OR 0.22; 95% CI: 0.21-0.24) decreased. Intermediate chronotype was associated with fewer sleep problems (median MOS-SPI-II = 27.2, p < 0.01), compared with definite morning (28.9) and evening types (31.7). This study shows that chronotype is associated with sleep-wake times in a work-free period, shift type preference, and sleep problems in nurses. Future studies on the association of shift work-induced circadian disruption and health outcomes should therefore consider chronotype as effect-modifier.