American researchers sought to estimate how many dementia cases each year might be associated with insomnia. The resulting figure is comparable to the population of a large city: nearly 450,000 cases per year in the United States alone. The study was published in The Journals of Gerontology: Series A (JGSA).

The team re-analyzed data from 5,899 adults over the age of 65. The dataset included self-reported insomnia symptoms as well as indicators of “probable dementia,” based on cognitive tests and medical diagnoses.

Researchers calculated the “population-attributable fraction” — a measure estimating the proportion of cases in the population that could theoretically be prevented if a specific risk factor were eliminated.

According to their calculations, 12.5% of dementia cases in older adults may be linked to insomnia. This means that if insomnia could be completely eliminated as a risk factor, approximately 449,000 dementia cases could have been prevented in the U.S. in 2022.

In the analyzed sample, 28.7% of participants reported insomnia symptoms, and 6.6% showed signs of probable dementia. Since the data represent a single time point, the researchers combined it with previous studies showing that insomnia increases the risk of developing Alzheimer’s disease by an average of 1.51 times.

Importantly, insomnia is considered a modifiable risk factor — unlike age or genetics, sleep problems can be treated and managed.

The authors note that this estimate is comparable to other known dementia risk factors. For instance, prior studies found that moderate to severe hearing loss in older age accounted for 16.9% of preventable cases, while mild hearing loss accounted for 3.9%. The impact of insomnia falls between these values.

The study also found that the contribution of insomnia to dementia risk is slightly higher in women than in men, with the largest number of potentially related cases occurring in people over 85 years old.

The relationship may be bidirectional: sleep disturbances can promote dementia development, while early brain changes in dementia may themselves cause insomnia, complicating the determination of a precise mechanism.

Nevertheless, the authors emphasize that these findings highlight the importance of assessing and treating sleep disorders in dementia prevention programs and standard geriatric practice.

“Considering sleep quality could play a key role in reducing dementia risk at the population level,” the researchers concluded.