Introduction The aim of the present study was to evaluate the association between chronotype and age in day and rotating shift workers.

Methods The present cross-sectional study was carried out between October 2012 and February 2015 in a large German chemical company. Employees participating in a regular voluntary occupational health check-up were requested to complete a written questionnaire, which included items on sleeping behaviour extracted from the Munich Chronotype Questionnaire. Inclusion criteria comprised a fully completed questionnaire, not having used an alarm clock on free days, and being employed either as a day or rotating shift worker. Senior executive managers, trainees and interns were excluded. We used univariable and multivariable linear regression analyses to assess the association between age and chronotype (in minutes) in the whole sample and stratified by shift status.

Result Altogether 10 348 persons completed the questionnaire, of which 4040 (39.0%) met the inclusion criteria. Participants were on average 41.8 years old (SD: 10.2), mainly male (75.4%) and engaged in day work (82.3%). Mean chronotype was 03:23 (SD: 54 min.) in the total sample, 03:16 (SD: 55 min.) in day and 03:57 (SD: 35 min.) in rotating shift workers. With increasing age, chronotype declined from 04:00 (≤29 years) to 03:08 (≥50 years) in the whole sample, and from 03:54 to 02:59 in day and 04:25 to 03:45 in rotating shift workers. Univariable and multivariable linear regression analyses correspondingly showed a significant decline of chronotype with age in both, day and rotating shift workers.

Discussion While day workers could benefit from a chronotype decrease, rotating shift workers could build up an intolerance regarding night work with increasing age. Shift workers might benefit from specific targeted prevention programs including sleep hygiene trainings.