The results, published in the medical journal of the American Academy of Neurology, suggest stress negatively affects memory and thinking skills.
Levels of cortisol, a hormone which helps the body respond to stress, were measured in 2,231 people with an average age of 49 over nearly a decade.
Participants were asked to take a psychological exam and assessments for memory and cognitive skills at the beginning of the study and were tested again eight years later.
Researchers took a blood sample from each participant to measure cortisol levels in the blood, which were then used to separate people into low, middle and high-level groups.
The results showed people with high levels of cortisol had lower scores on tests for memory and thinking skills than those with normal levels.
Nearly all of the participants (2,018) were also given an MRI brain scan to measure brain volume.
Researchers found the average “total cerebral brain volume” was lower in high-level participants compared to those with normal levels of cortisol.
"Cortisol affects many different functions so it is important to fully investigate how high levels of the hormone may affect the brain," said Justin B Echouffo-Tcheugui of Harvard Medical School.
Mr Echouffo-Tcheugui, who authored the study, added: "Our research detected memory loss and brain shrinkage in middle-aged people before symptoms started to show, so it's important for people to find ways to reduce stress, such as getting enough sleep, engaging in moderate exercise, or asking their doctor about their cortisol levels and taking a cortisol-reducing medication if needed.
“It’s important for physicians to counsel all people with higher cortisol levels.”
High levels of cortisol can be caused by stress, medical conditions or medication.
The findings come after the UK’s Mental Health Foundation (MHF) called for societal change earlier this year to combat stress in working life.
“Stress is one of the great public health challenges of our time, but it still isn’t being taken as seriously as physical health concerns,” said MHF director Isabella Goldie.
“We need to change at a societal level,” she said. “This includes ensuring that employers treat stress and mental health problems as seriously as physical safety.”
In the cortisol study, researchers acknowledged that the results had limitations, noting cortisol levels in the blood were only measured once and the findings may not be representative of long-term exposure to the hormone.
They also noted participants were mostly of European ancestry so results may not reflect the entire population.
Register for free to continue reading
Registration is a free and easy way to support our truly independent journalism
By registering, you will also enjoy limited access to Premium articles, exclusive newsletters, commenting, and virtual events with our leading journalists
Already have an account? sign in
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies