Are chronotype, social jetlag and sleep duration associated with health measured by Work Ability Index?

The present study investigates the impact of chronotype, social jetlag and sleep duration on self-perceived health, measured by Work Ability Index (WAI), within an industrial setting. Between 2011 and 2013, 2474 day and shift workers participated in a health check offered by an occupational health promotion program and filled out the Munich ChronoType Questionnaire (adapted to the rotational 12-h schedule for shift workers) and the WAI. We computed sleep duration on work and free days, chronotype, and social jetlag. We used linear regression models to examine chronotype, sleep duration and social jetlag for association with the WAI sum score, and proportional odds models to estimate the combined effect of social jetlag and sleep duration. Participants reported an average daily sleep duration of 7.35 h (SD: 1.2 h), had an average chronotype of 3:08 a.m. (SD: 1 h), and the average social jetlag corresponded to 1.96 h (SD: 2.05 h). Increasing social jetlag and shorter sleep duration were independently associated with a decreasing WAI, while chronotype per se was not associated with WAI. Short sleep duration combined with high social jetlag significantly increased the risk of poor WAI (OR = 1.36; 95% CI: 1.09-1.72), while long sleep duration and high social jetlag were not associated with poor WAI (OR = 1.09; 95% CI: 0.88-1.35). Our results add to a growing body of literature, suggesting that circadian misalignment, but not chronotype per se, may be critical for health. Our results indicate that longer sleep may override the adverse effects of social jetlag on WAI.

Primary selection into shift work and change of cardiovascular risk profile.

OBJECTIVES: A potential "healthy shift worker effect" may bias the studied effect of shift work on health. The observed differences among shift and day workers in health behavior and health outcomes can be caused by: (i) primary selection, (ii) the influence from the shift work-related environment, and (iii) the impact of shift work. We aimed to study these potential sources. METHODS: A cohort of 4754 male trainees who had finished their professional training and started their career in production in a chemical company between 1995 and 2012 was identified. Among them, 1348 (28%) were involved in rotating shift work and 3406 (72%) in day work. Information on health behavior and risk factors for cardiovascular diseases was retrieved from the medical examinations. This information was then compared (i) at the beginning of training, (ii) at the end of training, and (iii) 3 years after the employment, in relation to the working time. RESULTS: At the beginning of the training, the prevalence of smokers was higher among future shift workers (26% versus 21%), from 1995 to 2012. During the training and the first three years of employment, a marginal decline of systolic blood pressure and an elevation of triglyceride were related with shift work. No difference was found with respect to other risk factors for cardiovascular diseases. CONCLUSIONS: Our findings do not support a primary selection in favor of shift workers. An impact of shift work on the risk profile of cardiovascular diseases was not indicated in the observation period.

Shift work and risk of non-cancer mortality in a cohort of German male chemical workers.

OBJECTIVES: Shift work is widely considered to be a health risk. In a previous study, we observed no elevated risk of total mortality in BASF shift workers followed up until the end of 2006. The present study aims to investigate non-cancer mortality, especially mortality caused by ischaemic heart disease (IHD), relative to shift work. METHODS: The cohort consisted of 14,038 male shift and 17,105 male day workers from manufacturing plants, who were employed for at least 1 year between 1995 and 2005. Vital status was followed from 2000 to 2009. Cause-specific mortality was obtained from death certificates. Non-cancer mortality as well as mortality specific to diagnoses from I20.0 to I25.9 according to International Classification of Disease version 10 was compared between the two working-time systems. To estimate the impact of shift work on the outcome of interest, Cox proportional hazard model was used to adjust for potential confounders such as age, smoking, alcohol consumption, job level, and disease status at baseline. The effect estimates were then given as hazard ratio (HR) with 95 % confidence interval (CI). RESULTS: Between 2000 and 2009, a total of 1,062 deaths occurred in the cohort: 513 (3.6 %) in shift and 549 (3.2 %) in day workers. Among them were 122 deaths resulting from IHD, 55 (0.39 %) and 67 (0.39 %), respectively. After adjustment for age at entry and job level, no increased risk of non-cancer mortality (HR 0.94; 95 % CI 0.77-1.15) as well as of IHD-caused mortality was found among shift workers (HR 0.77; 95 % CI 0.52-1.14). The risk estimates were robust after further adjustment for more factors in all models and consistently tended to be in favour of shift workers. Considering the duration of exposure to shift, no dose-response relationship was found. CONCLUSION: The present analysis does not find strong evidence for an increased mortality risk due to non-cancer disease and, more specifically, IHD-caused mortality associated with this shift system. Initial selection based on health criteria as well as ongoing health surveillance and health promotion is likely to have contributed to this result. Shift work over 34 years may lead to a loss of this initial selection advantage over time, but the respective risk estimates lacked statistical precision.