That is according to research from the Finnish Institute of Occupational Health published in the American Journal of Epidemiology.
The study of 2,214 British civil servants found that middle aged employees who worked for over 55 hours a week also had worse short term memory and were less able to recall words than those who worked less than 41 hours.
Commenting on the results, 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 researchers went on to suggest that employees who work long hours are putting themselves as at much risk of developing dementia as those who smoke.
A number of companies including KMPG have asked their staff to volunteer to undertake a four day week to prevent them from having to make redundancies.
