What’s the Connection Between Daytime Napping and Alzheimer’s?

BrightFocus-funded researcher Peng Li, PhD wanted to know if sleep disruption and a malfunctioning “biological clock” are related to Alzheimer’s. Now he and colleagues have discovered that excessive daytime napping may be a sign.

In a new study recently published in the journal Alzheimer’s & Dementia (2022), Li and colleagues report on the results of their long-term study of more than 1,400 older adults, which points to a link between excessive daytime napping and the risk of developing Alzheimer’s disease (AD). The authors also found that participants with an AD diagnosis were more likely to nap during the day.

These results are some of the strongest evidence to date that circadian clock disruption and autonomic nervous system dysfunction may be part of the Alzheimer’s disease process – which is the key hypothesis behind Dr. Li’s 2020-23 BrightFocus Alzheimer’s Disease Research (ADR) grant, which helped fund these recent findings.

“Our results not only suggest that excessive daytime napping may signal an elevated risk of Alzheimer’s dementia, but they also show that faster yearly increase in daytime napping may be a sign of deteriorating or unfavored clinical progression of the disease,” he recently told the Harvard Gazette.

Dr. Li is an assistant professor of medicine at Harvard Medical School, based at Brigham & Women’s Hospital in Boston. Joining him as a co-author on the same study was former BrightFocus Alzheimer’s Disease Research program grant recipient Kristine Yaffe, MD, PhD, an epidemiologist, neurologist, and psychiatrist at the University of California, San Francisco.

More About the Circadian Clock – and Study Details

Humans and other living organisms possess inner biological, or “circadian” clocks that generate circadian rhythms on a 24-hour basis. These rhythms, or biological patterns, are sensitive to light and other parts of our environment, and they help drive physical, mental, and behavioral changes over a 24-hour day-night cycle. Among other things, circadian rhythms strongly influence our autonomic nervous system — a control system that acts largely unconsciously and regulates bodily functions such as heart rate and blood supply to the brain.

For long it’s been known that a diagnosis of Alzheimer’s disease (AD) often comes on the heels of years of sleep disturbances, and people with dementia are more likely to wake up during the night and struggle to return to sleep. Plus, conditions that disrupt sleep, such as sleep apnea, are more common in people diagnosed with AD. It’s still not clear, however, whether the disease disturbs sleep first or if changes to sleep quantity and quality can contribute to the development of AD.

Most research that assesses the effects of sleep on cognition focuses on nighttime sleep, and the studies that do examine daytime napping’s contributions to the development of dementia have relied on the reports of participants over a short time. In contrast, “our study calls for a closer attention to 24-hour sleep patterns — not only nighttime sleep but also daytime sleep — for health monitoring in older adults,” Dr. Li told the Gazette.

He and his colleagues followed 1,401 people for a long time— up to 14 years —as part of the Rush Memory and Aging Project. Participants wore a watch-like device on their wrists for up to two weeks. The device tracked their movement and activity, and those patterns of activity allowed researchers to draw conclusions about the individuals’ day and nighttime sleep habits.

The researchers found that as people aged, they napped more often and for longer periods. They determined that these longer, more frequent naps increased the risk of being diagnosed with AD. Plus, after receiving a diagnosis of AD based on clinically recognizable symptoms, participants tended to nap more often and for longer.

The findings suggest that the relationship between sleep and AD could go both ways. That is, in what the authors describe as a “vicious cycle,” excessive daytime napping may increase the risk for AD and conversely AD may lead to more daytime napping.

Past research has produced mixed results concerning the effects of daytime napping on cognition in older adults; it’s a question that needs to be further explored. Whereas some studies have shown that daytime napping has benefits on acute cognitive performance, mood, and alertness, other studies have highlighted adverse effects on cognitive performance.

The authors acknowledge that their findings could be limited by the older age of the participants in the study, who had an average age of 81, and their methods. Tracking activity via the device doesn’t necessarily mean that people were sleeping – just that they were lying quietly for a period of time.

Additionally, for these authors, next steps will be to monitor sleep using other methods and to test interventions that encourage people to modify their daytime sleep habits. Only then will these researchers be able to fully assess whether there is a causal link between napping and the risk of developing AD and dementia.

“A cure for AD is still lacking,” Dr. Li noted in his 2020 grant proposal. If his research is able to establish sleep disruption – and underlying autonomic dysfunction—as a contributor to AD, he believes it would lay the groundwork for the design of novel unobtrusive, cost-efficient tools for long-term monitoring of cognitive impairment or risk for AD. That would highlight the importance of circadian health and “encourage people to maintain regular daily schedules including sleep/wake in order to reduce AD risk and promote health aging,” he said.

In a word, it could lead to a new strategy for AD prevention. In addition, recognizing autonomic dysregulation as a contributor to AD could lead to better therapies and treatments for frequent AD complications, such as depression, dizziness, syncope (low blood pressure resulting in temporary loss of consciousness), and falls. This, in turn, could improve AD outcomes in the elderly—lowering related morbidity, institutionalization, and death rates.