While many workers dream of a four-day working week, new research suggests that the over 40s actually perform best while working three days a week.

A survey of 6,500 workers over the age of 40 in Australia, conducted by Melbourne Institute of Applied Economics and Social Research., found that working fewer hours helped to maintain a balance of productivity and keeping stress and exhaustion to a minimum.

Participants were asked to read aloud, recite backwards, and match letters and numbers within a strict timeframe. The study found that those who worked around 25 hours per week performed best and received the highest scores.

The report said: "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."

One of the participants, Colin McKenzie, professor of economics at Keio University, said longer working hours lead to reduced brain function, even claiming that those who work 60 hours per week have lower cognitive ability than unemployed people.

However, the findings have been called into question, given that the research only focuses on the over 40s, giving no indication as to whether the findings apply only to that age bracket.

Geraint Johnes, professor of economics at Lancaster University Management School and associate of Lancaster University's Work Foundation, 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.

"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."