The research looked at the impact of five-day weeks on cognitive ability, analysing 5,500 men and women.
Participants were asked to complete tests which investigated their work habits, memory, executive reasoning, and abstract reasoning.
The researchers found that, after 25 hours, performance declined in both male and female volunteers.
Worse still, it was found that volunteers who worked for 55 hours more has worse cognitive results than those who were retired or unemployed.
'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,' study co-author Colin McKenzie told The Times.
The moral of the story? Work less! It's good for your health.