Working overtime is bad for the heart, say results from a long-running study of more than 10,000 civil servants.

The research found that people who worked three or more hours longer than a normal seven-hour day had a 60 percent higher risk of heart-related problems such as death due to heart disease, non-fatal heart attacks and angina.

Marianna Virtanen, epidemiologist at the University College London (UCL), said: "The association between long hours and coronary heart disease was independent of a range of risk factors that we measured at the start of the study, such as smoking, being overweight, or having high cholesterol."

The Whitehall II study started in 1985 and recruited 10,308 office staff aged 35-55 from 20 London-based civil service departments.

Data has been collected at regular intervals and in the third phase, between 1991-1994, a question on working hours was introduced.

This current analysis looks at the results from 6,014 people (4,262 men and 1,752 women), aged 39-61, who were followed until 2002-2004, which is the most recent phase for which clinical examination data are available.

During the average 11.2 years of follow-up, Virtanen and her colleagues in Finland, London and France, found that there had been 369 cases of fatal CHD, non-fatal heart attacks (myocardial infarctions) or angina.

After adjusting for factors such as age, sex, marital status and occupational grade, they found that working three to four hours overtime (but not one to two hours) was associated with a 60 percent higher rate of CHD compared with no overtime work.

Further adjustments for a total of 21 risk factors made little difference to these estimates.

The researchers say there could be a number of possible explanations for this association between overtime and heart disease, said an UCL release.

Their results showed that working overtime was related to type A behaviour pattern (type A behaviour tend to be aggressive, competitive, tense, time-conscious and generally hostile), psychological distress manifested by depression and anxiety, and possibly with not enough sleep, or not enough time to unwind before going to sleep.