Circadian rhythms that are weaker, more fragmented, or that peak later in the day are associated with a higher risk of dementia, according to a study published on 29 December 2025 in Neurology, the medical journal of the American Academy of Neurology. The researchers found that people whose daily activity patterns were less clearly defined, or whose activity levels peaked later rather than earlier in the afternoon, were more likely to develop dementia over time. Notably, the findings show an association rather than proof that disrupted circadian rhythms directly cause dementia.
Circadian rhythms are the body’s internal timekeeping system. They regulate the 24-hour sleep–wake cycle and influence many other processes, including hormone release, digestion, and body temperature. The brain controls these rhythms and is strongly affected by exposure to light. When circadian rhythms are strong, the body clock stays closely aligned with the natural day–night cycle, producing clear and consistent signals that help regulate daily activity and rest.
People with robust circadian rhythms tend to keep relatively stable sleep and activity schedules, even when their routine changes slightly or the seasons shift. In contrast, weaker circadian rhythms are more easily disrupted by changes in light exposure or daily schedules. Individuals with weaker rhythms are more likely to experience shifts in their sleep and activity patterns, which may place additional strain on the body over time.
“Changes in circadian rhythms occur with ageing, and growing evidence suggests that circadian disruption may be a risk factor for neurodegenerative diseases such as dementia,” said study author Wendy Wang, MPH, PhD, of the Peter O’Donnell Jr. School of Public Health at UT Southwestern Medical Center. She explained that the study directly measured rest–activity patterns and found that participants with weaker, more fragmented rhythms, as well as those whose activity peaked later in the day, had an elevated risk of developing dementia.
The study followed 2,183 adults with an average age of 79 who did not have dementia at the beginning of the research. About 24% of participants were Black, and 76% were white. Each participant wore a small heart monitor attached to the chest for around 12 days, allowing researchers to measure periods of activity and rest continuously. Participants were then followed for an average of three years, during which time 176 people were diagnosed with dementia.
Researchers analysed several measures of circadian rhythm strength, including relative amplitude, which reflects the contrast between a person’s most active and least active periods. Higher relative amplitude indicates a stronger circadian rhythm. When participants were divided into groups, those with the weakest rhythms had nearly 2.5 times the risk of dementia compared with those with the strongest rhythms, even after accounting for factors such as age, blood pressure, and heart disease.
The study also found that people whose activity peaked later in the afternoon, from around 2:15 p.m. onwards, had a 45% higher risk of dementia than those whose activity peaked earlier. A later activity peak may indicate a mismatch between the body’s internal clock and environmental cues like daylight and darkness.
The researchers noted that circadian disruption may affect inflammation, sleep quality, and the brain’s ability to clear amyloid, a protein linked to dementia. However, a limitation of the study was the lack of data on sleep disorders, such as sleep apnoea, which could also influence dementia risk.
More information: Wendy Wang et al, Association Between Circadian Rest-Activity Rhythms and Incident Dementia in Older Adults The Atherosclerosis Risk in Communities Study, Neurology. DOI: 10.1212/WNL.0000000000214513
Journal information: Neurology Provided by American Academy of Neurology
