Working more than 11 hours a day rather than the usual 9am to 5pm markedly increases heart disease risk, say UK experts.
The magnitude of risk goes up by 67% for people who work long hours, they say in Annals of Internal Medicine.
The University College London team base their findings on over 7,000 civil service employees whose health they have been tracking since 1985.
They suggest GPs should now be asking their patients about working hours.
Lead researcher Professor Mika Kivimäki said: "Considering that including a measurement of working hours in a GP interview is so simple and useful, our research presents a strong case that it should become standard practice.
"It could also be a wake-up call for people who overwork themselves, especially if they already have other risk factors."
Over the course of the 11-year study, 192 of the participants suffered a heart attack.
People who worked 11 hours or more a day were more than half as likely again to have a heart attack than those who worked shorter hours.
And adding working hours to well-established heart risk factors, such as high blood pressure, made the researchers' predictions far more accurate.
If GPs were to add this to their usual list of heart questions they might spot 6,000 more of the 125,000 people who suffer heart attacks in the UK each year, the researchers suggest.
Studies are now needed to see if getting people to cut back on their working hours will improve their heart health, they add.
Professor Stephen Holgate of the Medical Research Council, which part-funded the investigation, said: "This study might make us think twice about the old adage 'hard work won't kill you'.
"Tackling lifestyles that are detrimental to health is a key area for the MRC, and this research reminds us that it's not just diet and exercise we need to think about."
Professor Peter Weissberg of the British Heart Foundation said: "These most recent findings raise the possibility that long working hours may increase the risk of a heart attack.
"But further studies are required to confirm this association and clarify how it might be used to change our current approach to assessing someone's risk of developing heart disease and what advice we give on working conditions."
Experts suspect a number of underlying factors may be at play, such as undetected high blood pressure, stress, anxiety or depression, and being a driven, aggressive or irritable personality.