Becoming much sleepier during the day in very old age may be linked to a higher chance of developing dementia.
A study of women in their 80s found that those whose overall sleepiness increased over several years were more likely to later receive a dementia diagnosis.
Dementia is a general term for a decline in memory, thinking and other mental abilities that becomes serious enough to interfere with daily life. Alzheimer’s disease is the most common cause, but several diseases and changes in the brain can lead to dementia.
Sleep has long interested brain researchers because it gives the brain and body time to recover. Sleep patterns also change naturally with age, and older people may sleep more lightly, wake more often at night or take more daytime naps.
Researchers led by Dr. Yue Leng at the University of California, San Francisco, wanted to know whether changes in sleep over time were connected with later problems in thinking and memory. They studied 733 women whose average age was 83 and who did not have dementia or mild cognitive impairment when the study began.
The women were followed for five years. At the beginning and end of the study, they wore small wrist devices for three days that recorded movement and helped researchers estimate nighttime sleep, daytime naps and daily sleep patterns.
The researchers identified three main patterns. About 44% of the women had fairly stable sleep, while 35% developed poorer nighttime sleep along with somewhat more napping and less regular sleep patterns.
The remaining 21% became increasingly sleepy over time. These women slept more during both the day and night and also showed greater changes in their normal daily sleep and wake cycle.
During the study, 164 women developed mild cognitive impairment, a condition in which memory or thinking problems are noticeable but are not severe enough to be classified as dementia. Another 93 women developed dementia.
The differences between the sleep groups were striking. Dementia developed in about 8% of women whose sleep stayed stable, compared with 15% of those whose nighttime sleep declined and 19% of those who became increasingly sleepy.
After researchers considered factors including age, education, race, diabetes and high blood pressure, increasing sleepiness remained important. Women in this group had about twice the risk of developing dementia compared with women whose sleep patterns remained stable.
The study does not show that extra sleep or daytime napping causes dementia. It is possible that early changes in the brain related to dementia affect sleep years before clearer memory and thinking problems appear.
Interestingly, worsening nighttime sleep by itself was not clearly linked to a higher dementia risk after other factors were considered. The stronger signal came from a broader increase in sleepiness, including more sleeping during both daytime and nighttime hours.
The findings could eventually help researchers understand whether changing sleep patterns are an early warning sign of declining brain health. However, sleepiness can have many causes, including medicines, sleep disorders, depression and other medical conditions, so it should not be treated as proof that dementia is developing.
The research was published on March 19, 2025, in Neurology, the medical journal of the American Academy of Neurology. The study was supported by the U.S. National Institutes of Health and the National Institute on Aging.
The researchers also noted that most participants were white women, so the results may not apply equally to men or to people from different racial and ethnic backgrounds. Larger and more diverse studies are needed to understand the connection and determine whether monitoring changes in sleep could help identify dementia risk earlier.
If you care about brain health, please read studies about dietary strategies to ward off dementia, and how omega-3 fatty acids fuel your mind.
For more health information, please see recent studies about Choline deficiency linked to Alzheimer’s disease, and what to eat (and avoid) for dementia prevention.
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