Feeling sleepier during the day as we get older may be more important than many people realize.
A new study suggests that older women who become increasingly sleepy over time may have a much higher chance of developing dementia.
The research does not prove that extra sleep causes dementia, but it shows a strong connection between changing sleep habits and declining brain health.
The study was published in the journal Neurology on March 19, 2025. Researchers wanted to understand how sleep changes in very old adults and whether those changes could help predict future memory and thinking problems. Scientists have long known that good sleep supports brain health, but they are still learning how sleep patterns change during aging.
The research team followed 733 women with an average age of 83 years. At the beginning of the study, none of the women had dementia or mild cognitive impairment. The researchers monitored them for five years to see how both their sleep and brain health changed.
At the start and end of the study, each participant wore a small device on her wrist for three days. The device measured how long they slept at night, how often they napped during the day, and how regular their daily sleep-wake cycle was. These measurements allowed the researchers to group the women according to how their sleep changed over time.
The largest group, representing 44% of participants, had stable sleep or slightly better sleep by the end of the study. Another 35% experienced poorer nighttime sleep, including shorter sleep and lower sleep quality, together with a small increase in daytime napping. The remaining 21% became much sleepier, sleeping more during both the day and the night while also developing more irregular daily sleep patterns.
Five years later, 164 women had developed mild cognitive impairment and another 93 had developed dementia. Dementia affected only 8% of women whose sleep remained stable. The rate increased to 15% among women whose nighttime sleep worsened and reached 19% among those whose daytime and nighttime sleepiness increased over time.
After taking into account differences such as age, education, race, diabetes, and high blood pressure, the researchers found that women with increasing sleepiness had about twice the risk of developing dementia compared with women whose sleep remained stable. This suggests that growing daytime sleepiness may be a useful warning sign that deserves medical attention.
Lead author Dr. Yue Leng from the University of California, San Francisco, said sleep plays a vital role in keeping the brain healthy by supporting memory and clear thinking. She explained that scientists still need to learn more about how sleep changes during aging and how these changes may influence dementia risk.
An interesting finding was that poorer nighttime sleep by itself was not strongly linked to dementia. Instead, the greatest risk was seen in women who became increasingly sleepy overall. This may mean that excessive sleepiness during the day, especially when combined with longer sleep at night, could reflect early changes happening in the brain before dementia becomes obvious.
The researchers noted that most participants were White, so future studies should include more diverse populations. The study was supported by the U.S. National Institutes of Health and the National Institute on Aging.
Although more research is needed, regularly noticing major changes in sleep habits and discussing them with a healthcare professional may help identify people who could benefit from earlier assessment and support. Monitoring sleep could become another useful way to protect brain health as people grow older.
If you care about dementia, please read studies about Vitamin B9 deficiency linked to higher dementia risk, and flavonoid-rich foods could help prevent dementia.
For more health information, please see recent studies that cranberries could help boost memory, and how alcohol, coffee and tea intake influence cognitive decline.
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