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The impact of chronotype on melatonin levels among shift workers

Objectives The association between shift work and cancer, which is thought to be mediated by effects on circulating melatonin levels, may be modified by chronotype (ie, the inherent preference for activity in the morning or the evening); however, few studies have examined the potential impact of chronotype on the carcinogenic effects of shift work. The authors analysed the impact of chronotype on previously reported differences in melatonin levels among healthcare workers that exclusively worked night or day shifts. Methods The cross-sectional study included 664 men and women (310 day shift and 354 night shift workers) from which urine samples were collected throughout work and sleep periods and were assayed for 6-sulfatoxymelatonin. Participants also completed the Composite Scale of Morningness, a questionnaire used to assess chronotype. Results Among both morning and evening-type night shift workers, 6-sulfatoxymelatonin levels were constitutively lower during daytime sleep, night-time sleep and night work compared with day shift workers during night-time sleep. However, morning-type night shift workers consistently showed 6-sulfatoxymelatonin levels that were closer to levels in day shift workers than did evening-type night shift workers. Differences in 6-sulfatoxymelatonin levels between morning-type and evening-type night shift workers relative to day shift workers were statistically significant in every instance (p<0.05). Conclusions These results suggest that morning-type night shift workers may be better able to maintain a ‘normal’ circadian pattern of melatonin production as compared with evening-type night shift workers. The impact of this chronotype effect on cancer risk among shift workers requires further study.