Doctors often recommend “REM sleep” as a way to compensate for a poor night’s sleep and help keep you alert until bedtime. But for older people, long naps can be an early sign of dementia.
Research on how daytime sleep affects cognitive function in adults has produced mixed results. Some research on young people suggests that sleep is good for cognition, while other research on older people suggests that it may be related to cognitive impairment. However, many studies are based on only one self-reported daytime nap. This methodology may not be accurate for people with cognitive impairment who cannot reliably report when and how long they slept.
As an epidemiologist studying sleep and neurodegeneration in the elderly, I wanted to find out if changes in daytime sleep habits portend other signs of cognitive decline. A study that my colleagues and I recently published showed that while the amount of daytime sleep increases with age, excessive sleep may herald cognitive decline.
The link between daytime sleep and dementia
Sleep disturbance and daytime sleep are known symptoms of mild to moderate Alzheimer’s disease and other forms of dementia in the elderly. They often become more extreme as the disease progresses: patients fall asleep less often and are more likely to wake up at night and feel sleepy during the day.
To explore this link between daytime sleep and dementia, my colleagues and I studied a group of 1,401 older adults with a mean age of 81 who participated in the Rapid Memory and Aging Project, a longitudinal study that studies cognitive decline and Alzheimer’s disease. Participants wore a watch-like device that tracked their mobility for 14 years. Long periods of inactivity were interpreted as daytime sleep.
At the start of the study, approximately 75% of the participants had no cognitive impairment. Of the remaining participants, 4% had Alzheimer’s disease and 20% had mild cognitive impairment, a common precursor to dementia.
While the number of daily naps increased among all participants over the years, there were differences in daytime nap habits between those who developed Alzheimer’s disease by the end of the study and those who did not. Participants who did not develop cognitive impairment had an average of 11 extra minutes of sleep per year. This rate doubled after the diagnosis of mild cognitive impairment, in which sleep increased to 25 extra minutes per year, and tripled after the diagnosis of Alzheimer’s disease, with sleep increased to 68 extra minutes per year.
Ultimately, we found that older people who slept at least once or more than an hour a day were 40% more likely to develop Alzheimer’s disease than those who did not sleep daily or slept less than an hour a day. These results did not change even after we controlled for factors such as daily activities, illness, and medications.
Sleep and the Alzheimer’s Brain
Our research shows that longer sleep is a normal part of aging, but only to a certain extent.
Research by my colleagues at the University of California, San Francisco offers a potential mechanism for why people with dementia sleep more often and longer. Comparing the post-mortem brains of people with Alzheimer’s with those of people without cognitive impairment, they found that people with Alzheimer’s had fewer neurons that promote wakefulness in three areas of the brain. These neural changes appear to be related to tau tangles, a hallmark of Alzheimer’s disease, in which a protein that helps stabilize healthy neurons forms clumps that prevent communication between neurons.
While our study does not show that increased daytime sleep causes cognitive decline, it does indicate that prolonged sleep is a potential signal of accelerated aging. Further research could determine whether nap monitoring can help detect cognitive decline.
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