Stress is a normal part of life. But what happens when the body’s stress system remains disturbed for years? A new study published in JAMA Network Open suggests that the way the stress hormone cortisol changes throughout the day may be linked to how the brain ages in older adults.
Researchers studied 3,895 older adults who were part of the long-running Chicago Health and Aging Project. The participants had an average age of about 77 years, and researchers followed their cognitive health over time. The study examined both Black and White participants, allowing scientists to investigate whether cortisol patterns and their relationship with cognition differed between the groups.
Cortisol: the body’s built-in stress messenger
Cortisol is often called the “stress hormone”, but that description is only part of the story. It is produced through the body’s hypothalamic-pituitary-adrenal (HPA) axis and helps regulate stress responses, metabolism and other essential functions. Normally, cortisol follows a daily rhythm. Levels tend to rise around the time we wake up and gradually fall as the day progresses. Think of it as the body’s biological timetable for stress and alertness.
Instead of measuring cortisol only once, researchers collected saliva samples at different times of the day like morning, afternoon and evening. Saliva is useful because it contains free cortisol, the biologically active form of the hormone, and collecting it is relatively simple and non-invasive. The researchers looked at several characteristics of the participants’ cortisol patterns, including the average amount of cortisol, total daily exposure and how much cortisol changed during the day.
One important finding was that people with higher cumulative cortisol exposure showed faster cognitive decline. In other words, consistently greater exposure to cortisol was associated with a steeper drop in cognitive performance over time. At the same time, the relationship was not simply “more cortisol equals worse cognition.” The researchers found that moderate day-to-day variation in cortisol was associated with slower cognitive decline, highlighting that the body’s stress system is more complicated than a simple high-versus-low measurement. Interestingly, the study did not find significant associations between the measured diurnal cortisol changes and the development of Alzheimer disease during the follow-up period. The researchers therefore describe salivary cortisol primarily as a possible early marker of neurocognitive aging, rather than a diagnostic test for Alzheimer’s disease.
Why this matters
The findings add another piece to the puzzle of brain aging. Scientists already study factors such as amyloid and tau proteins in Alzheimer’s disease. Cortisol could represent a different biological pathway that is one connected to long-term stress physiology. However, this was an observational cohort study. It shows an association, not proof that higher cortisol causes cognitive decline. Other biological, behavioral and social factors may also play a role.
Still, the results raise an interesting possibility: a simple saliva sample collected at different times of the day might eventually help researchers understand how stress-related biology is connected to brain aging. As scientists continue searching for earlier signs of cognitive decline, sometimes the clues may be hiding in places as ordinary as a small saliva sample.
Reference
Ng, T. K. S., Beck, T., Sudhini, Y., Wilson, R. S., Evans, D. A., & Rajan, K. B. (2026). Salivary cortisol and cognitive decline and Alzheimer disease in older adults. JAMA Network Open, 9(7), e2622955. https://doi.org/10.1001/jamanetworkopen.2026.22955
