Shift work, chronotype, and melatonin rhythm in nurses

Background: Previous studies associated night-shift work with melatonin disruption, with mixed evidence regarding the modulating effects of chronotype (i.e., diurnal preference). Methods: One hundred and thirty active nurses (84 rotating-shift and 46 day-shift workers) in the Nurses' Health Study II wore a head-mounted light meter and collected spontaneous urine voids over 3 days. 6-Sulfatoxymelatonin (aMT6s), the major urinary metabolite of melatonin, was assessed. Results: Rotating-shift workers on night shifts had more light exposure and lower urinary melatonin levels during the night, and urinary melatonin rhythms with smaller peaks [11.81 ng/mg-creatinine/h, 95% confidence interval (CI), 9.49–14.71 vs. 14.83 ng/mg-creatinine/h, 95% CI, 11.72–18.75] and later peak onset (5.71 hours, 95% CI, 4.76–6.85 vs. 4.10 hours, 95% CI, 3.37–4.99), compared with day-shift workers. Furthermore, evening chronotypes' melatonin rhythms had later peak onset compared with morning types (4.90 hours, 95% CI, 3.94–6.09 vs. 3.64 hours, 95% CI, 2.99–4.43). However, among day-shift workers, morning chronotypes had melatonin rhythms with greater mean levels, larger peaks, and earlier peak onset compared with evening chronotypes; patterns were similar comparing evening versus morning chronotypes among rotating-shift workers on night shifts. The interaction of rotating-shift work and chronotype was significant across all parameters (P < 0.05). Conclusions: As expected, rotating-shift workers on night shifts had greater light exposure and lower urinary melatonin levels during the night compared with day-shift workers. Intriguingly, melatonin rhythms were dependent on both chronotype and rotating-shift work type, and better alignment of rotating-shift work and chronotype appeared to produce less disrupted melatonin rhythms. Impact: The joint effects of shift-work type and chronotype require attention in future studies.