A three-day-week gets the best performance from workers aged over 40, a study has found.
Researchers found the cognitive performance of middle-aged people improved as the working week increased up to 25 hours a week.
However, when the week went over 25 hours, overall performance for the test subjects decreased as "fatigue and stress" took effect.
The report, which was published in the Melbourne Institute Worker Paper series, invited 3,000 men and 3,500 women in Australia to complete a series of cognitive tests while their work habits were analysed.
It was found those working 25 hours a week performed best while those working 55 hours a week showed results worse than retired or unemployed participants.
One of the three authors, Professor Colin McKenzie from Keio University told the Times: "Many countries are going to raise their retirement ages by delaying the age at which people are eligible to start receiving pension benefits. This means that more people continue to work in the later stages of their life.
"The degree of intellectual stimulation may depend on working hours. 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."
The research comes amid moves to edge the British state pension age closer to 70.
Currently for someone born in 1989, a state pension begins at the age of 68.
The government, however, will now review the pension age every five years with many people working past the formerly default retirement age of 65 out of choice or necessity.
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