People above the age of 40 should ideally limit their working week to between 20 and 30 hours, equivalent to a three-day week, researchers have suggested. A study by economics professors in Japan found that working beyond 30 hours can negatively impact cognitive functions, while working too little means people would not get the intellectual stimulation required to keep their brains active.

The study, titled "The effect of working hours on cognitive ability," looked at 2,965 men and 3,502 women aged 40 years and above. The research was undertaken by three economics professors from three Japanese universities – Shinya Kajitani of Meisei University, Colin McKenzie of Keio University and Kei Sakata of Ritsumeikan University.

The researchers studied samples from the Household, Income and Labour Dynamics in Australia Survey to understand how working hours affect a person's cognitive functioning. Participants underwent tests for memory span and cerebral dysfunction that involved a reading test, reading list of numbers backwards, and linking letters and numbers in a particular pattern within a specified time.

Based on their scores, the researchers concluded that "working hours up to 25–30 hours per week have a positive impact on cognition for males ... and up to 22–27 hours for females". Working beyond these hours has a negative impact on cognitive functioning.

McKenzie was quoted by The Times as saying that their research has come at a crucial time, when many countries are looking to extend the retirement age. "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," he said.

The research did not find "any statistically significant gender differences in the effects of working hours on cognitive functioning", indicating that the effects of long working hours are the same on men and women.

The research was published in the Melbourne Institute Working Papers series of the Melbourne Institute of Applied Economic and Social Research.