Could the body’s internal clock quietly shape the risk of dementia later in life? New research suggests the answer may be yes, adding weight to growing concerns that disrupted daily rhythms are not just a nuisance but a potential warning sign for brain health.
Circadian rhythm describes the body’s natural 24‑hour cycle, a biological clock that governs when we feel alert, sleepy, hungry, or ready to rest. It coordinates countless processes, from hormone release to body temperature.
When this rhythm runs smoothly, sleep and activity follow a fairly predictable pattern across the day. When it becomes irregular, the effects can ripple through physical and mental health.
A recent study published in Neurology, the journal of the American Academy of Neurology, has taken a closer look at this issue. Researchers focused on rest and activity rhythms, known as RARs, which act as measurable behavioural signals of circadian function. Their findings point to a striking link between disrupted daily rhythms and a higher risk of developing dementia in older adults.
The study examined how patterns of rest and movement across the day relate to the likelihood of new dementia cases emerging over time. Rather than relying on self‑reported sleep diaries or questionnaires, the research team used wearable medical devices. Participants wore ambulatory cardiac monitoring patches on their chests, typically used in heart rhythm assessment. These devices also captured movement data, offering a window into daily activity and rest cycles.
Each participant wore the patch for at least three days, though many wore it for far longer. On average, monitoring lasted about 12 days. This extended period allowed researchers to build a detailed picture of how consistent, strong, or fragmented each person’s daily rhythm was.
The group studied consisted of older adults with an average age of 79. Participants were followed for a median period of just over three years. During that time, around 8 per cent developed dementia. By comparing rhythm patterns at baseline with later health outcomes, researchers were able to identify which features of circadian disruption were most strongly linked to dementia risk.
Several key markers stood out. One was rhythm strength, which reflects how clear and pronounced the difference is between periods of activity and rest. A strong rhythm means active days and restful nights. A weak rhythm suggests blurred boundaries, with less distinction between day and night behaviour.
The analysis showed that weaker rhythm strength was strongly associated with a higher risk of dementia. For every standard deviation decrease in rhythm strength, the risk of developing dementia rose by more than 50 per cent. This was one of the most striking findings of the study.
Another important factor was rhythm fragmentation. This describes how often rest and activity are interrupted within the same day. Highly fragmented rhythms often manifest as daytime sleepiness, frequent naps, or waking repeatedly during the night. Each standard deviation increase in within‑day fragmentation was linked to a nearly 20 per cent increase in dementia risk.
Timing also mattered. People whose peak activity occurred later in the day, rather than in the morning or early afternoon, faced a substantially higher risk. Those with later peak activity times were almost 70 per cent more likely to develop dementia than those with more typical activity patterns.
The researchers also examined day‑to‑day consistency, known as interdaily stability. This measures how similar a person’s routine is from one day to the next. The results here were more complex. A U‑shaped pattern emerged, suggesting that both very low and very high consistency could be associated with increased risk. However, this association did not reach statistical significance in the main analysis.
To test the robustness of their findings, the team conducted several sensitivity analyses. These additional checks confirmed that participants with the weakest rhythms had the highest dementia risk. Importantly, many associations remained even after excluding individuals who already showed signs of mild cognitive impairment at the start of the study. This suggests that disrupted rhythms may precede, rather than simply reflect, early cognitive decline.
Taken together, the results paint a consistent picture. Alterations in circadian rest and activity rhythms, particularly weaker rhythm strength, greater fragmentation, and later peak activity, appear to be linked with a higher risk of incident dementia.
While causality cannot be established, the work strengthens the case that circadian biology plays a meaningful role in brain health during ageing.
At the same time, experts have highlighted several limitations. The study did not directly measure sleep quality or diagnose sleep disorders such as sleep apnoea or insomnia. This matters because disrupted sleep has previously been linked to increased production of amyloid, a protein associated with Alzheimer’s disease, and reduced clearance of this protein from the brain.
Including objective sleep data alongside activity rhythms could have provided deeper insight into the mechanisms at play. Without it, some uncertainty remains about whether the observed effects stem from circadian disruption itself, from poor sleep, or from a combination of both.
There are also questions about generalisability. All participants were older adults, leaving open the question of whether similar associations exist in middle‑aged populations, when preventive interventions might be more effective.
Another challenge lies in the technology used. Although the ambulatory cardiac patches provided valuable movement data, these devices were not designed specifically for circadian assessment. Some experts have noted that they have not been fully validated for this purpose, unlike wrist‑worn actigraphy devices commonly used in sleep research.
Differences in device placement also complicate comparisons with earlier studies. Previous research has often relied on hip or wrist monitors. Chest‑worn patches may capture movement differently, potentially affecting rhythm estimates.
The researchers also acknowledged the possibility of reverse causality. Subtle, undiagnosed brain changes could disrupt circadian rhythms before dementia is clinically apparent. Some dementia cases may have been missed during follow‑up, and certain health data relied on participant self‑reporting.
Physical activity was another factor not directly adjusted for in the analysis. Reduced movement due to depression, frailty, or other health conditions could influence both rhythm patterns and dementia risk, creating potential confounding effects.
Despite these caveats, the study opens new avenues for research and clinical practice. One intriguing aspect is the use of wearable cardiac monitors. These devices are already widely used in routine care. If movement data from such monitors can reliably flag circadian disruption, they could serve a dual purpose, identifying individuals at higher risk of cognitive decline without additional testing.
The findings also fuel interest in circadian‑focused interventions. Light therapy, structured daily routines, regular sleep schedules, and timed physical activity have all been proposed as ways to strengthen circadian rhythms. While evidence remains limited, these approaches are relatively low‑risk and could offer broader health benefits.
Researchers emphasise that more work is needed before such strategies can be recommended specifically to reduce dementia risk. Large, long‑term studies are required to determine whether improving circadian rhythms can actually alter the course of cognitive ageing, rather than simply reflecting underlying disease processes.
Future research may also explore whether circadian disruption and dementia influence each other in a bidirectional way. Brain changes could weaken daily rhythms, which in turn might accelerate neurodegeneration, creating a vicious cycle.
For now, the message is one of awareness rather than alarm. Regular patterns of activity and rest appear to be more than a matter of comfort or habit. They may offer valuable clues about brain health in later life.
As science continues to unravel the links between the body’s internal clock and cognitive decline, paying attention to daily rhythms could become an important part of healthy ageing.
