Cancer Epidemiol Biomarkers Prev

Background:Previous studies associated night-shift work with melatonin disruption, with mixed evidence regarding the modulating effects of chronotype (i.e., diurnal preference).Methods:130 active nurses (84 rotating-shift and 46 day-shift workers) in the Nurses\u2019 Health Study II wore a head-mounted light meter and collected spontaneous urine voids over three 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 (14.83, 95% confidence interval [CI], 11.72\u201318.75 versus 11.81 ng/mg-creatinine/hour, 95%CI, 9.49\u201314.71) and later peak onset (4.10, 95%CI, 3.37\u20134.99 versus 5.71 hours, 95%CI, 4.76\u20136.85), compared to day-shift workers. Further, evening chronotypes\u2019 melatonin rhythms had later peak onset compared to morning types (4.90 hours, 95%CI, 3.94\u20136.09 versus 3.64 hours, 95%CI, 2.99\u20134.43). However, among day-shift workers, morning chronotypes had melatonin rhythms with greater mean levels, larger peaks, and earlier peak onset compared to 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