People who work for more than 60 hours a week, and miss out on sleep, are far more likely to have a heart attack, claims research.

The study, carried out by Japanese researchers and, says that the combination could raise blood pressure and heart rate and trigger an attack.

However, other experts say that much more work needs to be done to prove a definite link.

UK workers toil for some of the longest hours in Europe, and the country has some of the highest rates of heart disease.

Many risk factors for the disease are known - such as smoking and a poor diet.

However, the potential interplay between these and other "psychosocial" factors, such as stress, mental illness and working habits are not fully understood.

The research, published in the journal Occupational and Environmental Medicine, involved hundreds of Japanese men, some who had suffered heart attacks, and some who had not.

The men who had had a heart attack worked significantly longer and slept less than those who had not.

This was a progressive increase - men working more than 60 hours a week had double the risk of heart attack compared with men work 40 hours or less.

No rest

An average night's sleep of five or fewer hours for just two nights a week doubled or even tripled the risk of a heart attack.

The results were adjusted to make sure that existing risks - such as obesity, high blood pressure and high cholesterol levels - did not unduly influence the findings.

Precisely why overwork increases heart attack risk is unknown, although it is possible that sharp increases in blood pressure caused by long hours combined with lack of sleep could trigger an attack.

Chronic stress has already been linked with abnormalities of heart function.

'More research needed'

A British expert, Dr Harry Hemingway, from University College London, said the case directly against long hours was not yet proven.

He told BBC News Online: "It is possible that depression is one factor not taken into account in this study.

"Certainly this could cause sleep disturbance, and long hours could also be the result of minor mental illness."

He pointed out that because the study relied on questionnaires filled in retrospectively by heart attack patients, their recollections of hours worked and sleeping patterns might not be completely reliable.

He said that more, larger-scale studies would be needed to prove the theory that long hours might contribute to heart attacks.

Professor Jim Horne, from the Sleep Research Centre at Loughborough University, said it was more likely that sleeping less was a symptom of a problem likely to contribute to heart problems rather than be a direct cause.

He said: "In experiments on rats who were deprived of sleep until they died, none showed any signs of vascular disease."