A Japanese research study ensured that confounding factors such as obesity, high blood pressure and high cholesterol levels did not influence study results. Reasons for the higher risk are not fully understood, but sharp increases in blood pressure caused by long hours, compounded by lack of sleep, could trigger an attack.

If further research validates a link between long hours and heart disease, companies that regularly require employees to put in long hours could be exposing themselves to large legal liabilities, according to Dr. Alex Kerin of Circadian Technologies.

Also, companies that do not regulate the amount of overtime their employees work may see their health insurance costs rise due to a higher incidence of heart attacks (patients with coronary heart disease have just under $9,000 in health costs per year on average), according to Dr. Kerin. In many cases, he says, it's more cost-effective to hire additional staff than to allow existing employees to work themselves to death or serious illness.