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

Mean levels of the melatonin metabolite aMT6s were 10.9 ng/mg creatinine/hour in night workers compared to 15.4 in day workers.

• Night workers exhibited lower urinary aMT6s levels compared to day workers.
• The lowest aMT6s levels were found in night workers who preferred morning activity.
• Peak aMT6s production occurred 3 hours later in night workers than in day workers.
• A stronger phase delay in melatonin production was associated with higher light-at-night exposure and more nights worked.
• Diurnal preference and light exposure intensity modified the differences in melatonin levels and timing.

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