The prevalence of male rotating shift work correlates with reduced total fertility rate: an ecological study of 54,734 reproductive-aged males in 35 European countries between 2000 and 2015

Europe has the lowest fertility rate in the world, and 19 European countries are already experiencing negative population growth. Previous studies have shown that light-dark cycle misalignment induces damages to semen quality in animals, but whether shift work impairs male human fertility remains controversial. The diverse types of shift work, which may have distinct health impacts, have not been differentiated in the few previous studies. The present study aims to assess the impact of different male shift work types, i.e., rotating shift work (RSW), permanent shift work (PSW), and daily split shifts (DSS), on male fertility in European countries, based on the European Working Conditions Survey of 54,734 men and 52,457 women of reproductive (15-49 years) age from 35 countries conducted between 2000 and 2015. Year-specific total fertility rate (TFR) for each country was obtained from the World Bank. Mixed linear model was used to analyze the association between the prevalence of shift work types and TFR among the countries during the 15 years span, with adjustment for latitude, per capita GDP, working duration, working frequency, median age of men or women engaged in shift work, education level of men or women, and female shift-work condition in reproductive aged participants. The results showed that the average prevalence of RSW declined from 15.6% in 2000 to 9.8% in 2010, and then rose to 11.1% by 2015, while TFR went up from 1.45 in 2000 to 1.60 in 2010, and then decreased to 1.56 in 2015. A 10% increase of RSW prevalence was associated with a 0.114 decrease of TFR (95% CI: -0.167, -0.061, P < .001), indicating 11.4 fewer births per 100 women throughout lifetime. The association remained significant (β = -0.088, 95% CI: -0.160, -0.015, P = .020) after adjusting for confounding factors. However, no association was observed between TFR and the prevalence of male PSW (β = -0.011, 95% CI: -0.094, 0.073, P = .801) or DSS (β = 0.357, 95% CI: -0.171, 0.884, P = .189). In conclusion, male RSW, rather than PSW and DSS, may reduce TFR. Further studies are needed to validate our findings.