The study, published in Occupational and Environmental Medicine, examined the relationship between number of working hours and hours of sleep and the risk of acute myocardial infarction (AMI). Researchers Suminori Kono of Kyushu University in Japan and David Snashall of Guy's and St Thomas's NHS Trust in London found that the number of weekly working hours were related to progressively increased odds of AMI in the past year as well as in the past month, with a two-fold increased risk for employees working 60 hours or more compared to those working 40 hours per week. Short sleep times of five hours or less, two or more times a week, were associated with a two-to-three-fold risk of AMI
Researchers studied 260 Japanese men ages 40-79 who were admitted to hospitals with AMI from 1996-1998. The control group was 445 men, free from AMI, who were matched for age and residence. All answered questions about their work, relaxation habits and sleep patterns, as well as lifestyle, weight, diet, blood pressure and chronic health problems such as high cholesterol or diabetes.
Although all the participants had similar lifestyles and medical conditions, the group who suffered AMI worked longer hours, rarely relaxed and slept less than five hours a night significantly more often than the other group.
Kono and Snashall theorized that sleep deprivation led to elevated blood pressure while chronic stress caused heart function abnormalities, both of which could trigger AMI. The optimal work week is 40 hours, they concluded, and suggested employees who must work longer hours get more sleep and spend more time relaxing.