Disruption of the regulation of circadian rhythms is a known feature of aging. As for everything to do with our biochemistry, this disruption of the circadian clock is complicated. As a starting point, there isn’t just one clock. The brain runs clocks, the periphery runs more clocks, and they coordinate with one another via signaling. That coordination breaks down with age, because everything breaks down with age in one way or another, as damage and dysfunction accumulates. We can also discuss whether the various clock mechanisms that sense aspects of the environment function correctly in later life, whether the appropriate signaling is still generated in the right way, whether the receptors for those signals still operate correctly to generate the appropriate cell and tissue responses, and so forth. There are many points at which normal function can be eroded as damage mounts – and it clearly is eroded in the old.
Today’s open access paper presents a novel way of looking at how the disruption of the circadian clock may contribute to age-related disease, specifically the risk of neurodegenerative disease in this case. The researchers used study data on movement and heart activity to characterize individual variations in the resilience of the circadian clock to alterations in the environment. An older person who is more affected by the environment is said to have a weak clock, whereas one who is less affected by changes in the environment has a strong clock. A strong clock correlates with a lower risk of dementia. This says little about causation, of course. The same accumulating cell and tissue damage of aging may provoke weakness in the circadian clock at the same time as it contributes to neurodegeneration. That weak clocks correlate with risk of dementia may just be pointing out that people with a greater burden of damage are more impacted than those with less of a burden of damage.
Do our body clocks influence our risk of dementia?
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.
A new study involved 2,183 people with an average age of 79 who did not have dementia at the start of the study. 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.
Association Between Circadian Rest-Activity Rhythms and Incident Dementia in Older Adults
Aging is associated with changes in circadian rhythms. Rest-activity rhythms (RARs) measured using accelerometers are markers of circadian rhythms. Altered circadian rhythms may be risk factors of neurocognitive outcomes; however, results are mixed. This was a retrospective examination of data from the Atherosclerosis Risk in Communities (ARIC) study. ARIC participants who wore the a long-term continuous monitoring patch in 2016-17 for ≥3 days and were free of prevalent dementia were included. RARs were derived from investigational accelerometer data from the patch.
Of the 2,183 participants (age 79 ± 4.5 years), 176 (8%) developed dementia. The median follow-up time was 3 years, and the mean patch wear time was 12 days. After multivariable adjustment, each 1 standard deviation decrement in relative amplitude and 1-SD increment in intradaily variability were associated with 54% and 19% greater risk of dementia, respectively. Further research to determine whether circadian rhythm interventions can reduce dementia risk is warranted.
