The study, published in the American Journal of Epidemiology, involved analyzing the performance of 2,214 middle-aged British civil servants on a battery of thinking tests given in two time periods from 1997 to 2004.
The workers were given cognitive tests, and those who worked more than 55 hours per week scored lower on vocabulary and reasoning exams than people whose work weeks lasted 40 hours at most, the researchers report.
Long working hours were predictive of lower reasoning and lower vocabulary scores on follow-up tests at the end of the study period.
The results held true regardless of age, sex, marital status, education, occupation, income, physical diseases, psychosocial factors, sleep disturbances, and health risk behaviors, the researchers report.
"This study shows that long working hours may have a negative effect on cognitive performance in middle age," the researchers conclude.
Study lead author Marianna Virtanen, MD, of the Finnish Institute of Occupational Health in Helsinki, notes that long working hours are common worldwide and that up to 17% of employees in the European Union worked overtime in 2001.
"Long working hours have been found to be associated with cardiovascular and immunologic reactions, reduced sleep duration, unhealthy lifestyle, and adverse health outcomes," the authors write.
There's also increasing evidence that such midlife risks may play a role in later dementia , they write.
Long hours have been found to negatively affect cognitive performance, grammatical acumen, and alertness, the researchers say.
The study examined the relationship between working long hours and cognitive function over a five-year follow-up period.
"Compared with employees working 40 hours or less per week, employees working more than 55 hours had lower vocabulary scores," the authors write. "At follow-up, they had lower scores also on the reasoning test."
People working more than 55 hours per week scored lower on two of five cognitive function measurements, the researchers report.
"Decline in cognitive function has already been shown to be present among the middle-aged," the researchers write. "As mild cognitive impairment predicts dementia and mortality, the identification of risk factors for mild cognitive impairment in middle age is important."
The study's findings are significant, researchers say, because long hours cause cognitive impairment in middle-aged people similar in magnitude to smoking , a known risk factor for dementia.
By Bill Hendrick
Reviewed by Louise Chang
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