How much work is too much work?

According to a recent study from the Melbourne Institute of Applied Economic and Social Research, working full time after the age of 40 is too much, for your brain at least.

The research, which involved collecting data from 6,500 people, found that doing more than three days a week once you reach 40 can damage your ability to think.

While working up to 30 hours a week is good for the brains of over-40s, if you were to work 60 hours a week your cognitive ability would be worse than that of someone who didn’t work at all.

The researchers used data from more than 3,000 men and 3,500 women who completed the national Household Income and Labour Dynamics in Australia (Hilda) survey. The survey tested the ability to read words aloud, recite lists of numbers and match letters and numbers in a speed trial.

Testing reading is a measure of the “knowing” part of ability, lead author Prof Colin McKenzie told the Guardian.

The other two tests capture fluid intelligence – the “thinking” part of ability that includes memory, abstract reasoning and executive reasoning.

“In all three cases (cognitive tests) around 25 to 30 hours of work per week will maximize your cognitive skill,” McKenzie told ABC News in an interview. “And going for less hours or more hours reduces your cognitive skills.”

Someone who worked between 50 and 60 hours a week had cognitive function as low as someone who did not work at all. In order to maintain the same cognitive performance, researchers believe a part-time job that requires only 20 to 30 hours a week would be ideal.

This lowering of scores in those aged over 40 who work full-time doesn’t fit with the idea that working for longer helps people stay mentally sharp, or with the notion of “use it or lose it”.

The study did not examine quality of work or if your ability to think is preserved depending on the type of job you do.

It’s also not clear why working more than 30 hours is not good for your brain, while fewer hours is beneficial. McKenzie said work can be a double-edged sword. “While work can stimulate brain activity, long working hours can cause fatigue and stress, which potentially damage cognitive functions.

“Full-time work (40 hours a week) is still better than no work in terms of maintaining cognitive functioning, but it is not maximising the positive effects of work,” he says.

Though is is difficult to control for all the factors that might bias a study such as this, including choices around the hours worked, holidays taken and the type of work, it makes the idea of working full time until the age of 66, the current State retirement age – which is set to rise to 68 by 2028 – even less appealing.

(Additional reporting: Guardian service)

This article was amended on August 11th, 2016