Extreme tiredness and stress could be as bad for the brain as smoking, according to the study.
The report, published in the American Journal of Epidemiology, found middle-aged workers clocking up more than 55 hours a week have poorer mental skills, including short-term memory and ability to recall words, than those working fewer than 41 hours.
The findings suggest the long-term damage to the brain caused by excessive time at work has been underestimated.
One in eight British workers puts in more than the supposed European maximum of 48 hours a week.
Researchers, led by Dr Marianna Virtanen from the Finnish Institute of Occupational Health, monitored 2,214 British civil servants from the 1980s.
Participants in their early 50s were put through a series of brain function tests. Those doing the most overtime recorded lower scores in two of the five key brain function tests - reasoning and vocabulary.
The researchers said: "This study shows that long working hours may have a negative effect on cognitive performance in middle age. The link between cognitive impairment and dementia later in life is clearly established.
"The difference between employees working long hours and those working normal hours is similar in magnitude to that of smoking, a risk factor for dementia."
Professor Cary Cooper, an expert in workplace stress from Lancaster University, told the Daily Mail: "Working long hours obviously makes you very tired. If you do that on a consistent basis it's going to affect your brain function. Long hours are not just bad for health, they are also bad for your performance."
The Great Red Satain
The report, published in the American Journal of Epidemiology, found middle-aged workers clocking up more than 55 hours a week have poorer mental skills, including short-term memory and ability to recall words, than those working fewer than 41 hours.
The findings suggest the long-term damage to the brain caused by excessive time at work has been underestimated.
One in eight British workers puts in more than the supposed European maximum of 48 hours a week.
Researchers, led by Dr Marianna Virtanen from the Finnish Institute of Occupational Health, monitored 2,214 British civil servants from the 1980s.
Participants in their early 50s were put through a series of brain function tests. Those doing the most overtime recorded lower scores in two of the five key brain function tests - reasoning and vocabulary.
The researchers said: "This study shows that long working hours may have a negative effect on cognitive performance in middle age. The link between cognitive impairment and dementia later in life is clearly established.
"The difference between employees working long hours and those working normal hours is similar in magnitude to that of smoking, a risk factor for dementia."
Professor Cary Cooper, an expert in workplace stress from Lancaster University, told the Daily Mail: "Working long hours obviously makes you very tired. If you do that on a consistent basis it's going to affect your brain function. Long hours are not just bad for health, they are also bad for your performance."
The Great Red Satain
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