Working more than 60 hours a week and constant sleep deprivation may double the risk of having a heart attack, according to a survey conducted by researchers at Japan's National Cancer Center.
The study, published in the British medical journal Occupation and Environmental Medicine, looked at 260 Japanese men between the ages of 40 and 79 who had survived a first-time heart attack, comparing them with a group of 445 men with no heart attack history.
The researchers obtained details of the men's weekly working hours, the number of days off and the amount of sleep within the last month and during the past year.
They also looked at potential heart attack risk factors, including lifestyle, weight and conditions such as high blood pressure, high cholesterol and diabetes.
The results showed that men working more than 60 hours a week faced a double risk of a heart attack when compared with those who worked 40 hours or less a week.
Sleeping an average of five hours or less a night and having frequent lack of sleep, defined as two or more days a week of under five hours, were also associated with a doubling or tripling of the risk.
The authors, led by Ying Liu of the National Cancer Center in Tokyo, said: "Overtime work is known to increase blood pressure and heart rate, and induce cardiac or psychological symptoms such as chest pain, depression and fatigue."
The study, which ran from 1996 to 1998, suggests that the optimal working week is a maximum of 40 hours, and that those who work longer should ensure they get enough sleep and have at least two days of rest a month.