Highlights:
- A new study has found circadian rhythm, the body's internal clock, may affect a person's risk of dementia.
- More than 2,000 people wore monitors for an average of 12 days to track their rest and activity rhythms.
- Researchers found people with weaker or more irregular body clocks had a higher risk of developing dementia.
- Being most active later in the day, instead of earlier, was linked to a 45% increased risk of dementia.
- Future studies of circadian rhythm interventions, such as light therapy or lifestyle changes, could help determine if they can lower a person's risk of dementia.
MINNEAPOLIS — Circadian rhythms that are weaker and more fragmented are linked to an increased risk of dementia, according to a new study published on December 29, 2025, in Neurology® , the medical journal of the American Academy of Neurology . The study also found that circadian rhythm levels that peaked later in the day, rather than earlier, were also linked to an increased risk. The study does not prove that these factors cause dementia, it only shows an association.
Circadian rhythm is the body's internal clock. It regulates the 24-hour sleep-wake cycle and other body processes like hormones, digestion and body temperature. It is guided by the brain and influenced by light exposure.
With a strong circadian rhythm, the body clock aligns well with the 24-hour day, sending clear signals for body functions. People with a strong circadian rhythm tend to follow their regular times for sleeping and activity, even with schedule or season changes. With a weak circadian rhythm, light and schedule changes are more likely to disrupt the body clock. People with weaker rhythms are more likely to shift their sleep and activity times with the seasons or schedule changes.
"Changes in circadian rhythms happen with aging, and evidence suggests that circadian rhythm disturbances may be a risk factor for neurodegenerative diseases like dementia," said study author Wendy Wang, MPH, PhD, of the Peter O'Donnell Jr. School of Public Health at UT Southwestern Medical Center in Dallas, Texas. "Our study measured these rest-activity rhythms and found people with weaker and more fragmented rhythms, and people with activity levels that peaked later in the day, had an elevated risk of dementia."
The study involved 2,183 people with an average age of 79 who did not have dementia at the start of the study. Of participants, 24% were Black people and 76% were white people.
Participants wore small heart monitors that adhere to the chest to measure rest and activity for an average of 12 days. Researchers used data from the monitors to track the strength and patterns of people's circadian rhythms. Participants were then followed for an average of three years and during that time 176 people were diagnosed with dementia.
Researchers reviewed heart monitor data for various measures to determine circadian rhythm strength. These measures included relative amplitude, which is a measure of the difference between a person's most active and least active periods. High relative amplitude signified stronger circadian rhythms.
Researchers divided participants into three groups, comparing the high group to the low group. A total of 31 of 728 people in the high group developed dementia, compared to 106 of the 727 people in the low group. After adjusting for factors such as age, blood pressure and heart disease, researchers found when compared to people in the high group, those in the low, weaker rhythm group had nearly 2.5 times the risk of dementia, with a 54% increased risk of dementia for every standard deviation decrease in relative amplitude.
Researchers also found people who experienced a peak of activity later in the afternoon, 2:15 p.m. or later, compared to earlier in the afternoon, 1:11 p.m.-2:14 p.m., had a 45% increased risk of dementia. Seven percent of those in the early group developed dementia, compared to 10% of those in the high group.
Having a later peak of activity means there could be a difference between the body clock and environmental cues such as later hours and darkness.
"Disruptions in circadian rhythms may alter body processes like inflammation, and may interfere with sleep, possibly increasing amyloid plaques linked to dementia, or reducing amyloid clearance from the brain," said Wang. "Future studies should examine the potential role of circadian rhythm interventions, such as light therapy or lifestyle changes, to determine if they may help lower a person's risk of dementia."
A limitation of the study was that researchers did not have information on sleep disorders, like sleep apnea, which could affect the results.
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The American Academy of Neurology is the leading voice in brain health. As the world's largest association of neurologists and neuroscience professionals with more than 40,000 members, the AAN provides access to the latest news, science and research affecting neurology for patients, caregivers, physicians and professionals alike. The AAN's mission is to enhance member career fulfillment and promote brain health for all. A neurologist is a doctor who specializes in the diagnosis, care and treatment of brain, spinal cord and nervous system diseases such as Alzheimer's disease, stroke, concussion, epilepsy, Parkinson's disease, multiple sclerosis, headache and migraine.
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