Older adults with the weakest 24-hour activity rhythms were roughly 2.5 times more likely to develop dementia than peers with the strongest rhythms, according to a study published December 29, 2025 in Neurology, the medical journal of the American Academy of Neurology.
The finding adds the strength of the body's internal clock, called the circadian rhythm, to the growing list of factors that appear to shape dementia risk. It is one of the first large prospective studies to link rhythm strength directly to who later gets diagnosed.
What the researchers measured
The team, co-led by Wendy Wang of UT Southwestern Medical Center's Peter O'Donnell Jr. School of Public Health and Lin Yee Chen of the University of Minnesota, followed 2,183 community-dwelling adults with an average age of 79. None had dementia at the start.
Instead of relying on sleep questionnaires, each participant wore a small chest-mounted ECG patch for an average of about 12 days. The device's built-in accelerometer captured movement, which the researchers used to derive rest-activity rhythms.
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Over an average of about three years, 176 participants developed dementia. After adjusting for age, education, cardiovascular factors and the APOE ε4 gene variant linked to Alzheimer's, several rhythm patterns stood out.
Every one standard deviation drop in relative amplitude, a measure of the gap between a person's most active and least active hours, was tied to a 54 percent higher risk of dementia. Those with the weakest overall rhythms had close to 2.5 times the risk of those with the strongest.
Timing mattered too. People whose activity peaked at 2:15 p.m. or later had a 45 percent higher dementia risk than those peaking between 1:11 and 2:14 p.m. In raw numbers, 10 percent of the later-peak group developed dementia versus 7 percent of the earlier-peak group.
What the study can and cannot say
This is observational research, so it cannot prove that a weaker rhythm causes dementia. It is possible that very early, still-silent brain changes disrupt the rhythm rather than the other way around. The researchers also caution that more work is needed before such monitoring could be used clinically.
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Still, the biological plausibility is there. "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," Wang said in the American Academy of Neurology announcement.
The stakes are large. The Alzheimer's Association estimates that 7.4 million Americans age 65 and older are living with Alzheimer's in 2026, or about 1 in 9 people in that age group.
What a strong body clock looks like in practice
Circadian rhythm is the body's internal 24-hour cycle that governs sleep, hormones, appetite and activity. A strong rhythm has a clear peak of activity during the day and a deep dip at night. A weak or fragmented rhythm looks flatter, with more nighttime movement, more daytime napping and a later shift in when the body is most active.
The researchers did not test any specific intervention. But Wang pointed to areas worth exploring in future work.
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"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," she said.
Sleep specialists generally recommend a few habits that support a stronger day-night contrast. Get bright light, ideally sunlight, in the morning. Keep bedtimes and wake times consistent, including on weekends. Move during the day, especially in the first half. Dim indoor light in the evening and keep the bedroom dark and cool.
Anyone noticing persistent memory changes, disrupted sleep or a new pattern of daytime confusion should discuss it with a doctor. The new research does not change treatment guidelines, but it does suggest that how the body organizes its 24-hour day may be worth paying more attention to as people age.
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