Regularly staying late at the office significantly increases the risk of having a stroke or heart attack, researchers have found.
Working just an hour extra each day pushes up stroke risk by 10 per cent, according to a huge review of working patterns.
And people who work an extra three hours a day - or 55 hours a week - are 33 per cent more likely to have a stroke, the scientists found.
The research, published today in the Lancet medical journal, is based on health records of more than 500,000 people across Europe, the US and Australia.
Scientists led by University College London warned that people who work longer hours tend to do less exercise, sit at a computer for hours at a time, and drink more than people with a better work-life balance.
Repeated stress may also be to be blame for greater health problems, the scientists think.
They looked at the combined result of 17 previous studies, in which 529,000 people were tracked over an average of seven years.
They found that people working 41 to 48 hours a week were 10 per cent more likely to suffer a stroke than those who worked a typical 35 to 40-hour week.
People who worked 49 to 54 hours a week had a 27 per cent increased chance of suffering a stroke.
And those who worked 55 or more hours were 33 per cent more likely to fall victim.
The researchers also looked at the link between hours worked and the risk of coronary heart disease, including heart attacks and angina.
Those who worked more than 55 hours a week had a 13 per cent increased risk of heart disease, they found.
The authors wrote: ‘Sudden death from overwork is often caused by stroke and is believed to result from a repetitive triggering of the stress response.
‘Behavioural mechanisms, such as physical inactivity, might also link long working hours and stroke; a hypothesis supported by evidence of an increased risk of incident stroke in individuals who sit for long periods at work.’
The EU Working Time Directive gives people the right to limit their average working time to 48 hours per week - but many employers and workers ignore the rule.
Lead author Professor Mika Kivimaki, from University College London, said: ‘The pooling of all available studies on this topic allowed us to investigate the association between working hours and cardiovascular disease risk with greater precision than has previously been possible.
‘Health professionals should be aware that working long hours is associated with a significantly increased risk of stroke, and perhaps also coronary heart disease.’
Experts last night stressed that the study had only found a link between hours worked and heart problems - it had not proved what had actually caused each death.
But they warned people to take note of the findings.
Dr Tim Chico, consultant cardiologist at Sheffield Teaching Hospitals, said: ‘For many people, reducing their working hours would be difficult or impossible, and this study does not show that it would reduce the risk of stroke.
‘Most of us could reduce the amount of time we spend sitting down, increase our physical activity and improve our diet while working and this might be more important the more time we spend at work.
‘We should all consider how the working environment could be altered to promote healthy behaviour that will reduce strokes, irrespective of how long we work.’
Dr Mike Knapton of the British Heart Foundation, which part-funded the study, said: ‘This study highlights to doctors that they need to pay particular attention to cardiovascular risk factors when they advise people who work long hours.’
Dr Shamim Quadir of the Stroke Association added: ‘Working long hours can involve sitting for long periods of time, experiencing stress and leads to less time available to look after yourself.’