People who work well beyond the standard 40-hour week may show a somewhat faster mental decline in middle-age, a new study suggests.

Researchers found that among more than 2,200 UK government employees, long work hours were linked to poorer performance on certain tests of cognitive function — as well as a steeper decline in mental acuity over time.

Compared with men and women who worked 35 to 40 hours per week, those who worked 55 hours or more showed a greater decline in reasoning ability over five years.

It's not clear whether the long work hours themselves explain the difference, according to lead researcher Dr. Marianna Virtanen, of the Finnish Institute of Occupational Health in Helsinki.

She and her colleagues accounted for a number of other factors — like workers' specific jobs, education and medical conditions like high blood pressure and heart disease. Still, Virtanen told Reuters Health, there may be other, unmeasured factors that account for the association between long work hours and poorer cognitive function.

Even if there is a "true effect," she noted, the decline associated with long work hours was mild.

The findings, published in the American Journal of Epidemiology, are based on a five-year follow-up of 2,214 middle-aged civil service workers. At the beginning and end of the study period, employees completed five standard tests of cognitive function.

In general, workers who logged 55-plus hours per week scored lower on one test — a vocabulary test — at the both the beginning and end of the study. They also showed a greater decline over time in a test of so-called fluid intelligence, which is related to a person's ability to reason and problem-solve.

Employees who worked long hours tended to have higher stress levels, sleep less and drink more than their counterparts who worked a standard week. However, that did not fully explain their lower cognitive-test performance, according to Virtanen.

For now, she said, the reasons for the link are unknown. "There remains a lot to be investigated in this topic," she said.