Determinants and methods of assessment of melatonin levels among rotating shift nurses

Abstract

Background: Long-term night shift work has been associated with multiple cancer sites, including breast, prostate, colon and endometrial. The mechanism for this effect is hypothesized to include the hormone melatonin; where increased light at night exposure during shift work reduces melatonin production and decreased melatonin levels are associated with increased cancer risk. In addition, physical activity has been shown to reduce cancer risk and existing laboratory studies indicate it has the potential to influence melatonin levels.

Methods: A cross-sectional study of light intensity exposure, physical activity and melatonin levels was conducted among 61 rotating shift nurses at Kingston General Hospital. Light intensity exposure was measured using a light intensity data logger and melatonin concentrations were measured from urine and saliva samples, collected over a 24-hour period. Physical activity was assessed from a study questionnaire and one-day diary.

Results: A statistically significant inverse association between light exposure and urinary melatonin levels was observed; however, the relationship was no longer significant when stratified by shift group. Analysis of salivary melatonin levels demonstrated that circadian rhythms of melatonin production in night workers were not altered in timing, such that peak melatonin production occurred at night. No association between light exposure and the magnitude of salivary melatonin variation was observed. The relationship between recent physical activity and melatonin differed by shift group, with a positive association seen among day workers, while an inverse relationship was seen among night workers. There was no association between usual physical activity and melatonin in either shift group. Finally, no significant correlation was observed between sleep duration and melatonin among either day or night workers.

Conclusions: While this study demonstrated an inverse relationship between light intensity and melatonin, the comparison of functional time points between day and night workers meant that differences in urinary melatonin levels between shift groups could be attributed to differences in the time of day when urine samples were collected. No consistent relationship between recent or usual physical activity and melatonin levels was observed in either shift group. Sleep duration was not correlated with urinary melatonin levels, suggesting it cannot be used as a proxy for melatonin production.