Feeling overworked? It may be because you’ve maxed out your brain’s capacity, according to a new report published by a team of researchers from the University of Melbourne in Australia and Keio University in Japan. By the time people hit their 40th birthday, working more than 30 hours a week becomes too much for their brains to handle.

"For cognitive functioning, working far too much is worse than not working at all," Colin McKenzie, an economics professor from Keio University, told The Sydney Morning Herald in an interview. “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."

For the study, McKenzie and his colleagues analyzed the work habits of 3,000 men and 3,500 women in Australia. Each participant underwent a cognitive exam to test their memory, reading, and ability to unscramble words that are spelled incorrectly, which provided insight into their baseline brain function. Researchers found participants who were over the age of 40 and worked more than 30 hours a week experienced a negative decline in cognitive ability. Women worked best from 22 to 27 hours a week, while men tapped out between 25 to 30 hours. After those hours, work left employees fatigued and stressed, which can potentially lead to cognitive damage, according to McKenzie.

“In all three cases [cognitive tests] around 25 to 30 hours of work per week will maximize your cognitive skill," McKenzie told ABC News. "And going for less hours or more hours reduces your cognitive skills. Too much work leads to stress and fatigue and that's probably the key cause of this decline in cognitive skills. So these chances to refresh your body and brain may be important in determining the optimum peak.”

They found that an overworked employee’s brain was the worst of them all. An employee who clocked in between 50 and 60 hours a week had cognitive function as low as someone who didn’t 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. Working excessively long hours lowers the quality of work, and our aptitude to work productively and efficiently. To stay mentally sharp at work, experts recommend establishing and maintaining close ties with others in and out of work. Both social interaction and a reliable support system can prevent or reduce the damage stress can have on the brain.

The researchers argue that delaying retirement age could actually worsen the brain’s ability to function — a decision many people make because of the financial necessity to save more money. According to the U.S. Department of Labor, as of May 2016, roughly 123.6 million Americans worked more than 35 hours a week.

But researchers found the work cap applied only to those over 40. So why don’t long work weeks have a negative effect on people in their 20s and 30s? According to McKenzie, the part of the brain that’s responsible for recovery may be a little more adaptable in the young than those in middle age. He continued. "Younger people are more resilient to working longer hours on a continued basis."

Source: McKenzie, Kajitani S, and Sakata K. Use It Too Much And Lose It? The Effect of Working Hours on Cognitive Ability. Melbourne Institute Working Papers Series. 2016.