Inside the body, a 24-hour rhythm, known as the circadian rhythm, quietly coordinates when we sleep, wake, eat and recover. This internal timing system helps keep organs and hormones working in sync. When it becomes disrupted, the effects may extend well beyond poor sleep, with growing evidence suggesting consequences for long-term brain health.

A large 2025 study of more than 2,000 people with an average age of 79 found that those with a strong circadian rhythm had an almost halved risk of developing dementia. Circadian rhythms regulate daily processes including sleep timing, hormone release, heart rate and body temperature.

Over three years of follow-up, dementia developed in 7% of participants with irregular body clocks, measured using heart rate monitors, compared with 10% of those whose rhythms remained more regular.

Disrupted circadian rhythms are often associated with poor sleep. For decades, poor sleep has been suspected to contribute to both dementia and heart disease, which share several underlying risk factors. In the 2025 study, worse heart health and high blood pressure, both commonly linked to sleep disruption, were accounted for in the analysis. Sleep apnoea, however, was not.

Sleep apnoea is a common condition in which breathing repeatedly stops and starts during sleep, reducing oxygen supply to the brain and raising blood pressure. Its relationship with dementia remains debated, largely because sleep apnoea is more common in people who already have established dementia risk factors such as obesity, diabetes, smoking and alcohol........

© The Conversation