Workers aged over 40 perform at their best if they work three days a week, according to economic researchers.
Their research analysed the work habits and brain test results of about 3,000 men and 3,500 women aged over 40 in Australia.
Their calculations suggest a part-time job keeps the brain stimulated, while avoiding exhaustion and stress.
The researchers said this needed to be taken into consideration as many countries raise their retirement age.
Data for the study was drawn from the Household, Income and Labour Dynamics in Australia survey, which is conducted by the Melbourne Institute of Applied Economics and Social Research at the University of Melbourne.
It looks at people's economic and subjective well-being, family structures, and employment.
Those taking part were asked to read words aloud, to recite lists of numbers backwards and to match letters and numbers under time pressure.
In general terms, those participants who worked about 25 hours a week tended to achieve the best scores.
"Work can be a double-edged sword, in that it can stimulate brain activity, but at the same time, long working hours and certain types of tasks can cause fatigue and stress which potentially damage cognitive functions," the report said.
Colin McKenzie, professor of economics at Keio University who took part in the research, said it would appear that working extremely long hours was more damaging than not working at all on brain function.
The figures suggest that the cognitive ability of those working about 60 hours a week can be lower than those who are not employed.
However, Geraint Johnes, professor of economics at Lancaster University Management School, said: "The research looks only at over-40s, and so cannot make the claim that over-40s are different from any other workers.
"What the authors find is that cognitive functioning improves up to the point at which workers work 25 hours a week and declines thereafter."
He added: "Actually, at first the decline is very marginal, and there is not much of an effect as working hours rise to 35 hours per week. Beyond 40 hours per week, the decline is much more rapid."