Circadian Variation of Melatonin, Light Exposure, and Diurnal Preference in Day and Night Shift Workers of Both Sexes

Background: Light-at-night has been shown in experimental studies to disrupt melatonin production but this has only partly been confirmed in studies of night shift workers. In this cross-sectional study we examined the circadian variation of melatonin in relation to shift status, individual levels of light-at-night exposure and diurnal preference, an attribute reflecting personal preference for activity in the morning or evening. Methods: 117 workers (75 night and 42 day) of both sexes, aged 22-64 years, were recruited from four companies. Participants collected urine samples from all voids over 24 hours and wore a data logger continuously recording their light exposure. Socio-demographic, occupational, lifestyle and diurnal preference information were collected by interview. Urinary 6-sulfatoxymelatonin, the main melatonin metabolite, concentrations were measured. Results: Mean 6-sulfatoxymelatonin levels were lower in night (10.9 ng/mg creatinine/h; 95% CI, 9.5-12.6) compared to day workers (15.4; 95% CI, 12.3-19.3). The lowest 6-sulfatoxymelatonin levels were observed in night workers with morning preference (6.4; 95% CI, 3.0-13.6). Peak time of 6-sulfatoxymelatonin production occurred 3 hours later in night (08:42 h, 95% CI 07:48-09:42) compared to day workers (05:36 h, 95% CI 05:06-06:12). Phase delay was stronger among subjects with higher light-at-night exposure and number of nights worked. Conclusions: Night shift workers had lower levels and a delay in peak time of 6-sulfatoxymelatonin production over a 24 hour period. Differences were modified by diurnal preference and intensity of light-at-night exposure. Impact: Night shift work affects levels and timing of melatonin production and both parameters may relate to future cancer risk.