Health problems associated with single, multiple, and the frequency of months of objectively measured long working hours: a cohort study by the National Institute of Occupational Safety and Health, Japan

PURPOSE: We aimed to examine the prospective associations of monthly working hours measured in a month, the 6-month averaged hours, and the frequency of long working hours (≥ 205 h/month) during the past 6 months with health indicators. METHODS: This study included 6,806 Japanese company workers (response rate = 86.6%). Data on the workers’ monthly attendance during the second half of fiscal year 2016 and annual health checkups in fiscal years 2016 and 2017 were collected. We evaluated the association of the above three types of monthly working hours with subsequent health checkup data in fiscal year 2017. We adjusted for the corresponding data in fiscal year 2016. RESULTS: Multivariate logistic regression analyses revealed significant associations between monthly working hours and workers’ systolic and diastolic blood pressure as well as aspartate aminotransferase, alanine aminotransferase, low-density lipoprotein cholesterol (LDL), and triglyceride levels. However, the associations were not consistent between months. The average monthly working hours were significantly associated with higher LDL levels for the 220–240 h/mo group (OR: 1.49, 95%CI: 1.07–2.08) and lower triglyceride levels for the < 140 h/mo group (OR: 0.15, 95%CI: 0.03–0.77), compared to the 140–180 h/mo group. The frequency of long working hours was significantly associated with higher LDL levels. CONCLUSIONS: Working hours over several months produced various associations with health indicators compared to those measured in a single month. Our present data suggest that the effects of average or frequency of long working hours during the past 6 months are likely to appear in LDL levels.