Working long hours Monday to Friday is tough no matter how old you are, but recent research suggests that, if you're over 40, working more than 25 hours a week could actually be affecting your cognitive performance.

After testing 6,500 workers in Australia, researchers found that workers had the best mental abilities when working around three days a week, but anything more than that was associated with a decline in memory and intelligence. So... anyone else want to call in sick tomorrow?

Interestingly, working less than 25 hours was also associated with low cognitive scores, which led the researchers to conclude that there's a sweet spot for mental stimulation above a certain age.

To find this out, an international team of researchers tested 3,000 Australian men and 3,500 women over the age of 40 in three categories: a memory score test, a reading test, and a perceptive ability test.

"In all three cases it was found around 25 to 30 hours of work per week will maximise your cognitive skill," one of the researchers, Colin McKenzie from Keio University in Japan, told ABC News.

"For cognitive functioning, working far too much is worse than not working at all," he told the Sydney Morning Herald. "In the beginning work stimulates the brain cells. The stress associated with work physically and psychologically kicks in at some point and that affects the gains you get from working."

So how can too much work be bad for us? The research only looked at the link between working hours and performance, and didn't investigate what could be causing this change, so it's too soon to say for sure that it was work doing the damage.

But the level of cognitive decline associated with long hours was the same in both men and women, and only the amount of time worked seem to explain the difference in test performance across the sample size.

The researchers hypothesise this is to do with two things - stress and a lack of sleep.

Stress has been known for decades to impact our cognitive function through the production of hormones - and research has even shown that prolonged stress can trigger the loss of neurons in the brain.

Lack of sleep has a similar effect, with studies showing that the white matter inside your brain can significantly change after a night of no sleep.

The researchers only looked into workers over the age of 40 in this research, so they can't say for sure how long working hours impact people who are younger.

But the team is particularly interested in that period of life, seeing as it's what McKenzie calls the "sandwich years", when people are often caring both for young children and older relatives - which means they have a job on top of a job.

Despite all that added pressure, retirement ages are being pushed back and the average working week is getting longer and longer in most developed countries - despite no evidence that it's actually making us any more productive.

The research was published in April as a working paper for the Melbourne Institute Working Paper Series, and more study is now needed to confirm exactly how long people should be working for optimum performance - and to examine how too much work affects us on a neurological level throughout our lives.

But in the meantime, maybe we need to take some inspiration from companies in Sweden, which are trialling a mandatory six-hour work day to get the most out of their workers.

Or Ontario, Canada, which is experimenting with paying people a set wage each month, just for being alive - no work required.

Let's hope some solid science will help restore a little bit of work/life balance to the world - because we definitely need it.