Employees over the age of 40 should work for no more than three days a week to ensure their minds stay sharp.

Japanese researchers proved part-time employment is the way forward once you hit middle-age by looking at the working conditions and performances of 3,000 men and 3,500 women in Australia.

The research, involving timed literacy and numeracy tests, showed that those who worked for 25 hours a week performed best.

People who worked for 40 hours a week performed slightly worse, while those who put in over 55 hours a week performed as badly as if they were unemployed or retired.

Colin McKenzie, an economics professor at Keio University and one of the paper's authors, said:

"Work can be a double-edged sword, in that it can stimulate brain activity, but at the same time long working hours can cause fatigue and stress, which potentially damage cognitive functions.

"We point out that differences in working hours are important for maintaining cognitive functioning in middle-aged and elderly adults.

"This means that, in middle and older age, working part-time could be effective in maintaining cognitive ability".