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"Karoshi" (translated as worked to death) is a concern in Japan. A paper published in 2006 showed that over a quarter of Japan's workforce were working 50 hours or more per week with half of that working 60 hours or more per week. The Ministry of Health, Labor, and Welfare launched an investigation in 2002 to examine 300 cases of possible Karoshi (death from being over-worked) during a time frame between 2002 and 2005. From their study, they recommended that employers offer a physician interview/guidance for any employee working over 100 hours overtime/month. Or if an employee requested a doctor's assessment.

Authors of one paper performed a metal analysis on four publish cohort studies and nineteen unpublished studies looking at the effect of "long working hours" (defined by the authors as 55 or more hours per week) to look for an association of developing Type II Diabetes compared to a referent group of workers only working 35-40 hours per week. They found that there was statistical significant evidence for as association between longer working hours and development of Type II Diabetes but only so for the low socioeconomic group such as manual laborers when compared to other SES groups. The lower SES group working longer hours had a 29% increased risk of developing Type II diabetes even after adjusting for physical activity, smoking, alcohol use, and obesity. However another author commented that residual confounding might account for this higher risk in the lower SES group compared to the higher SES group. They also postulated that sleep may be a mediator for this association since inadequate or poor quality and or quantity of sleep can be a predictor of Type II diabetes. The workers in the lower SES group may have less control over their schedule and this may include disruptive schedules interfering with circadian rhythm.

One large scale study using data from the Individual-Participant-Data Meta-analysis in Working Population Consortium comprised of 85, 494 workers from several European countries also looked at the effects of long work hours and the association with developing atrial-fibrillation. These participants were free of atrial fibrillation at baseline. Mean age was 43.4 years. During follow up over 10 years, those working long hours (55 hours or more per week) were at 40% higher risk of developing atrial fibrillation compared to those working a standard 35-40 hour-week. In their study, the investigators controlled for age, sex, SES, smoking, Body Mass Index, physical activity and alcohol consumption. The investigators also commented that this association between long work hours and atrial fibrillation appeared to be independent of classic risk factors of atrial fibrillation due to the similarities of the exposed group (long work hours) and the referent group.

For Coronary Heart Disease (CHD), a meta-analysis of four prospective studies published in 2012 found a 1.4 fold increased risk of CHD associated with long working hours. The investigators also noted that this association was higher for men than women. They hypothesized underlying mechanisms for this association may include longer exposure to stress, sleep deprivation, and/or dysregulation of the HPA axis causing an increase in cortisol production. They also noted that this estimate might have been biased for several reasons. One reason could have been if workers with underlying, diagnosed CHD reduced their working hours in the years before a CHD event. Another could be due to confounding of SES. Kivimaki et performed an additional meta-analysis adjusting for these factors and found the risk of a CHD event to be 1.13 instead. Kivimaki et al also found a 1.3 times higher risk for those working long hours with risk of stroke.