The stress of long hours may be a factor
Long working hours may raise the risk of mental decline and possibly dementia, research suggests.
The Finnish-led study was based on analysis of 2,214 middle-aged British civil servants.
It found that those working more than 55 hours a week had poorer mental skills than those who worked a standard working week.
The American Journal of Epidemiology study found hard workers had problems with short-term memory and word recall.
Lead researcher Dr Marianna Virtanen, from the Finnish Institute of Occupational Health, said: "The disadvantages of overtime work should be taken seriously."
It is not known why working long hours might have an adverse effect on the brain.
However, the researchers say key factors could include increased sleeping problems, depression, an unhealthy lifestyle and a raised risk of cardiovascular disease, possibly linked to stress.
The civil servants who took part in the study took five different tests of their mental function, once between 1997 and 1999, and again between 2002 and 2004.
Those doing the most overtime recorded lower scores in two of the five tests, assessing reasoning and vocabulary.
The effects were cumulative, the longer the working week was the worse the test results were.
Employees with long working hours also had shorter sleeping hours, reported more symptoms of depression and used more alcohol than those with normal working hours.
Professor Mika Kivimäki, who also worked on the study, said "We will go on with this study question in the future.
"It is particularly important to examine whether the effects are long-lasting and whether long working hours predict more serious conditions such as dementia."
Professor Cary Cooper, an expert in workplace stress at the University of Lancaster, said it had been long established that consistently working long hours was bad for general health, and now this study suggested it was also bad for mental functioning.
He said: "This should say to employers that insisting people work long hours is actually not good for your business, and that there is a business case for making sure people have a good work-life balance.
"But my worry is that in a recession people will actually work longer hours. There will be a culture of "presenteeism" - people will go to work even if they are ill because they want to show commitment, and make sure they are not the next to be made redundant."
Harriet Millward, deputy chief executive of the Alzheimer's Research Trust, said: "This study should give pause for thought to workaholics.
"We already know that dementia risk can be reduced by maintaining a balanced diet, regular social interactions and exercising both our bodies and minds. Perhaps work-life balance should be accounted for too."