To the best of our knowledge, this is the first study to investigate the association between long working hours and metabolic syndrome. This study suggests that compared with subjects who work 7–8
h/day, those reporting long working hours (>10
h/day) had over double the odds of metabolic syndrome after adjusting for age, occupation, shift work, smoking status, frequency of alcohol consumption, and cohabiting status. Similar patterns were observed among non-shift workers. In the stratified analysis by age-group, long working hours were associated with a higher risk of metabolic syndrome among older subjects, although no clear association was observed among younger subjects.
It should be noted that we observed no substantial increase in risk of metabolic syndrome among subjects who worked ≤10
h/day, although we observed a two-fold increase in odds among those who worked >10
h/day. Thus, the present findings suggest 10
h/day as a possible trigger level of working hours for metabolic syndrome among Japanese male workers. In line with the present findings, Kawakami et al. [13
] showed in their prospective cohort study of Japanese men that those who worked ≥50
h/day) had a 3.7 times higher risk of developing non-insulin-dependent diabetes than those who did not undertake overtime work. A six-year study of women in the Nurses’ Health Study II indicated that subjects who worked ≥41
h/week were associated with a higher risk of type 2 diabetes than those who worked 21–40
]. In contrast, Nakanishi et al. [15
] reported that among Japanese male workers, the relative risk of type 2 diabetes significantly decreased among those who worked >10
h/day compared with those who worked 7–8
h/day. Another case–control study in Japanese men has also shown that working >11
h/day is associated with more than double the risk of myocardial infarction compared with men whose mean working time was 7–9
]. By applying the benchmark dose approach, Suwazono et al. [31
] examined the benchmark durations of working hours for development of fatigue symptoms in Japanese workers. Assuming a condition of worst job stress, they demonstrated that the threshold number of working hours among men was approximately 10
h/day with a benchmark response of 5%. On the other hand, it can be argued from our findings that 8
h/day could also be a possible trigger level. Indeed, the point estimates of ORs in Models 1 and 2 were the highest among those who worked 8–9
h/day, and we observed a dip among those who worked 9–10
h/day in Models 1 to 3. The increased ORs among those who worked 8–9
h/day, however, could be partially explained by the effect of the Labour Standards Law, which stipulates that working hours be limited to 40
h/week. Workers who developed health problems may have been required to restrict their working hours to avoid further adverse health outcomes arising from long working hours. This so-called healthy worker effect could also have been induced by self-regulation among subjects who have higher control over their work schedule, and this tendency may be more pronounced in workplaces with a higher social network or social capital [32
]. Although the evidence remains inconclusive, our findings appear to suggest that a 10-hour working day is the trigger level for a significant increase in the risk of metabolic syndrome among Japanese male workers aged ≥40
One plausible explanation for the link between long working hours and metabolic syndrome may be insufficient recovery time, primarily caused by shortened sleeping hours as a result of the long working hours [20
]. Indeed, long working hours (≥40
h/week) have been shown to be a risk factor for the development of shortened sleeping hours (<6–7
], which may increase the risk of obesity [38
]. In addition, since short sleep duration has been found to be a risk factor for diabetes, hypertension, and all-cause mortality [41
], long working hours could increase the risk of chronic medical conditions by shortening sleep duration [42
]. A case–control study in Japan indicated that long working hours and sleep deprivation are associated with an increased risk of acute myocardial infarction [43
]. When we additionally adjusted for sleeping hours as a supplementary analysis, we observed a slight attenuation in the positive association between working hours and metabolic syndrome. Although careful consideration is required with its interpretation, this finding may support the above-mentioned hypothesis [44
Another plausible explanation for the present findings may be that long working hours could modify lifestyles, resulting in a lack of time or energy available for healthy activities. For example, a recent review showed that leisure-time physical activity was less common among those working more than 45–50
]. Indeed, lifestyle factors including physical activities are inter-correlated, and there may be complex interactions among them. A recent study in Japan reported that adherence to healthy lifestyles (such as regular physical activity and healthy eating) was associated with a lower risk of metabolic syndrome [47
]. Although information relating to such factors was not available in the present study, it is likely that lifestyle factors are influenced by the work environment, especially long working hours.
We observed heterogeneous effects of long working hours on metabolic syndrome across different age-groups, and a significant association was noted only among subjects aged 40
years or older. This finding may well reflect the long-term effects of long working hours among older subjects. Indeed, it is less likely that long working hours could increase the risk for metabolic syndrome in the short term. However, we should note that the present findings do not necessarily imply that long working hours among younger workers are permissible: even shortly after joining a company, long working hours may potentially influence the future risk of metabolic syndrome by inducing behavioral and lifestyle changes. We should also note that the prevalence of metabolic syndrome among subjects aged <40
years was lower than that among the subjects aged ≥40
years or older (6.8% and 14.8%, respectively), which is consistent with the findings of the National Health and Nutrition Survey in Japan [4
]. Among younger subjects, it may be that factors in operation before they start work at a company (e.g., family background and childhood lifestyle) determine the risk for metabolic syndrome. From a life-course perspective, future studies are warranted to investigate both pre- and post-employment risk factors (including working hours) for metabolic syndrome.
On a related issue, previous studies have reported that shift workers are at higher risk of developing hypertension and obesity [48
]. In addition, other cohort studies have reported an association between shift work and abnormal glucose metabolism [50
], which may suggest an association between shift work and metabolic syndrome. When we restricted the present analysis to non-shift workers, we observed similar patterns to those in the total sample. However, partly because of the small sample size, we were unable to assess the relationship among shift workers. Additional studies are needed to examine how specific aspects of work schedules, including the length and frequency of rotation, influence the risk of metabolic syndrome.
Interestingly, when we further examined the association between working hours and metabolic syndrome by restricting the analysis to technicians and skilled workers, we observed heterogeneous results with respect to occupation. Given the considerably low precision of the estimates, it is necessary to exercise caution when interpreting the results of this supplementary analysis. This finding may, however, indicate the significance of occupation when examining the association between working hours and metabolic syndrome. This point has been less carefully examined and, given the widening health disparities across occupations for both sexes during the recent economic downturns in Japan [52
], future studies are necessary to investigate this possible heterogeneous effect.
First, working hours were measured only at a single time point, and we could not assess the history of working hours among the subjects. It is likely that working hours vary substantially depending on economic conditions, and further studies with repeated measurement of working hours may yield important findings in this regard. Second, since working hours were assessed by self-reported questionnaire, the possibility of misclassification cannot be ruled out. However, the misclassification of working hours would be non-differential to metabolic syndrome, which may have attenuated the present findings. Third, owing to lack of information about the use of medication, the possibility of misclassification of diseased as non-diseased conditions cannot be ruled out. This misclassification, however, would be non-differential, which may have attenuated the present results. Fourth, because of the cross-sectional study design, we cannot exclude the possibility of reverse causation. However, it is implausible that the presence of metabolic syndrome caused the working hours of subjects to increase. Fifth, information about physical activities and job stress was not available. Since these factors potentially mediate the association between working hours and metabolic syndrome, their assessment would promote our understanding of this cause-effect relationship [54
]. Sixth, half of the study subjects were skilled workers (49.7%), which, despite the high response rate (96.2%), may reduce the generalizability of our findings to Japanese male workers as a whole.