"Objective The aim of this review was to synthesize the evidence on the potential relationship between nightshift work and breast cancer. Methods We searched multiple databases for studies comparing women in shift work to those with no-shift work reporting incidence of breast cancer. We calculated incremental risk ratios (RR) per five years of night-shift work and per 300 night shift increases in exposure and combined these in a random effects dose–response meta-analysis. We assessed study quality in ten domains of bias. Results We identified 16 studies: 12 case–control and 4 cohort studies. There was a 9% risk increase per five years of night-shift work exposure in case–control studies [RR 1.09, 95% confidence interval (95% CI) 1.02–1.20; I2=37%, 9 studies], but not in cohort studies (RR 1.01, 95% CI 0.97–1.05; I2 =53%, 3 studies). Heterogeneity was significant overall (I2=55%, 12 studies). Results for 300 night shifts were similar (RR 1.04, 95% CI 1.00–1.10; I2=58%, 8 studies). Sensitivity analysis using exposure transformations such as cubic splines, a fixed-effect model, or including only better quality studies did not change the results. None of the 16 studies had a low risk of bias, and 6 studies had a moderate risk. Conclusions Based on the low quality of exposure data and the difference in effect by study design, our findings indicate insufficient evidence for a link between night-shift work and breast cancer. Objective prospective exposure measurement is needed in future studies."

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