Association between Night-Shift-Work and Level of Melatonin: Systematic Review and Meta-Analysis

Abstract

Background: Night-shift workers are exposed to nocturnal light and are more prone to develop circadian rhythm disorders. Although night-shift work is thought to be associated with decreasing levels of melatonin, studies have showed inconsistent results. Methods: This systematic review and meta-analysis studied the association between night-shift work and levels of melatonin and its metabolite, 6-sulfatoxymelatonin (aMT6s). PubMed and EMBASE databases were searched up to August 30, 2019. The difference in melatonin levels between night-shift workers and day workers was statistically quantified by standardized mean difference (SMD) and 95% confidence interval (CI). Findings: Thirty-three studies reported in 25 articles (1,845 night-shift workers and 3,414 controls, mean age 45.12 years) were included after systematic literature review. Pooled results showed that first morning-void aMT6s level in night-shift workers was significantly lower than in day workers (SMD = -0.101, 95% CI= -0.179 to -0.022, P =0.012). The level of mean 24-hour urinary aMT6s was lower in night-shift workers than day workers (SMD: -0.264, 95% CI: -0.473 to -0.056, P = 0.013). Among fixed night-shift workers, the level of circulating melatonin, as well as first morning-void aMT6s was lower than that of day workers. Interpretation: The exposure of night-shift work is inversely associated with the levels of urinary aMT6s and blood melatonin. Night-shift workers experience to chronic reduction in melatonin which might introduce a high risk of carcinoma. Public health and medical services are needed to relieve the harm of night-shift work. Funding Statement: This work was supported by Natural Science Foundation of Shandong Province, China (No. ZR2017MH100). Declaration of Interests: The authors declare no conflicts of interest. Ethics Approval Statement: The study protocol was designed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) and was registered in the PROSPERO database.