People who work 11 or more hours a day have double the odds of becoming depressed compared with those who don't work overtime, a British study suggests.

The study in this week's issue of the journal PLoS One published by the Public Library of Science followed 2,123 middle aged civil servants for six years.

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Working excessive hours is associated with an increased risk of major depression, researchers say. (iStock)

Those who worked an average of at least 11 hours a day showed up to 2.5 times the risk of a major depressive episode compared with those working a standard seven- to eight-hour day, the researchers found.

All of the subjects were considered mentally healthy when they started the study. After six years, just over three per cent were diagnosed with clinical depression.

"Although occasionally working overtime may have benefits for the individual and society, it is important to recognize that working excessive hours is also associated with an increased risk of major depression," said Marianna Virtanen of the Finnish Institute of Occupational Health and University College London, who led the study.

Most of the civil servants, 52 per cent, worked a regular workday.  Another 37 per cent averaged nine- or 10-hour days, and 11 per cent clocked 11 hours or more.

Depression was found more often in junior and mid-level employees than those in senior positions.

The researchers said that taking into account factors such as physical disease, smoking, alcohol use, job strain and social support at work had little effect on the link between excess working hours and depression.

They speculated that long working hours may in part affect mental health through factors they didn't measure, such as work/family conflicts, difficulties in unwinding after work or prolonged increases in the stress hormone cortisol.

Since the study looked at white-collar civil servants, the findings may not apply to blue-collar workers or to the private sector.

The authors called for a different type of study to test whether reducing working hours could change depression risk among workers.

The study was funded by the British Medical Research Council, British Heart Foundation, Stroke Association, U.S. National Heart Lung and Blood Institute and the U.S. National Institute on Aging.