Objectives A potential "healthy worker effect" may bias the studied effect of shift work on health. The observed differ-ences in health behaviour and health outcomes might be caused by: (i) primary selection, (ii) influence from the with shift work related environment, and (iii) impact of shift work. We aimed to study these potential sources.

Method A cohort of 4754 male trainees who had finished their professional training and started their career in the production between 1995 and 2012 was identified. Among them, 1348 (28%) were involved in rotating shift work and 3406 (72%) in a day working time. Information on lifestyle and blood pressure (BP), body mass index, and measurements of laboratory parameters were retrieved from the data of medical examinations. These were then compared at (i) the beginning, (ii) the end of training, and (iii) 3 years after the job begin, in relation to the working time.

Results At the beginning of the training, the prevalence of smokers was higher among shift workers (26% vs. 21%). During the training and the first 3 years of job, marginal decline of systolic BP and elevation of triglyceride were observed among the shift workers, after the adjustment for age at begin, duration between the exami-nations, and the measurements at the baseline. No difference was found with respect to other risk factors for cardiovascular diseases.

Conclusions Our findings do not support a primary selection in favour of shift workers. A potential impact of shift work on lifestyle and health will be studied prospectively.