Sleepless Nights Could Drive Half a Million Cases of Dementia in The US Each Year

Scientists have attempted to put a number on how many dementia cases could be caused by insomnia each year – and their figure is similar to the population of a mid-sized US city.

Researchers from several US institutions reanalyzed existing data on 5,899 people aged 65 and over. This included self-reported insomnia symptoms and "probable dementia" based on cognitive decline tests and medical diagnoses.

The main takeaway was that 12.5 percent of dementia cases in older adults, which equates to nearly 450,000 people per year in the US alone, could be attributable to insomnia.

"Despite growing evidence of an association between insomnia and dementia, no studies have quantified the population-level impact of insomnia on dementia in the United States," write the researchers in their published paper.

The study builds on what we already know about insomnia's links to dementia. (Ben Blennerhassett/Unsplash)

This population-level impact is known as the population attributable fraction (PAF) – the proportion of illness or death in a population that can be attributed to a specific risk factor. In practical terms, the PAF estimates the disease burden that could be prevented if that risk factor were removed.

First, the team established how many people in their dataset had insomnia or probable dementia, which were 28.7 percent and 6.6 percent, respectively.

The dataset provided only a single snapshot; however, those individuals weren't followed over time to see which of them developed dementia. So these numbers were combined with a relative risk figure from previous studies, showing that, on average, having insomnia makes you 1.51 times more likely to develop Alzheimer's dementia.

The researchers then used an established formula to calculate PAF: extrapolating the snapshot data to represent the entire US, then using the relative risk multiplier to estimate the number of dementia cases that theoretically would not occur if insomnia were not a contributing factor.

That calculation produced a PAF of 12.5 percent, or roughly one in eight dementia cases. Applying that to everyone aged 65 and older with dementia in the US: "An estimated 449,069 dementia cases in 2022 could have been prevented if insomnia were eliminated," the authors write.

To calculate PAF, a cause-and-effect relationship needs to be assumed: in this case, that insomnia contributes to dementia. At the same time, PAF doesn't prove a causal relationship, because it's a mathematical model that makes several assumptions.

It's still very useful, though – it puts a real number on the potential impact of insomnia on dementia. Sleep problems are considered a modifiable risk factor, meaning they can be addressed by individuals and healthcare professionals.

"These estimates are notable when considered relative to other established dementia risk factors," write the researchers. "For example, Smith et al. reported a PAF of 16.9 percent for moderate or severe hearing loss and 3.9 percent for mild hearing loss in late life using the same data source."

"Our PAF estimate falls between these numbers, highlighting the significant population-level impact of insomnia and suggesting considerable potential for interventions aimed at reducing insomnia among older adults."

It's also worth bearing in mind that insomnia and dementia may have a two-way relationship, to some extent: It's possible that brain changes associated with dementia lead to sleeping problems, rather than the other way around.

That makes it tricky for researchers to tease out what's actually driving the progress of dementia, and what's happening as a consequence. New studies continue to identify multiple risk factors, which get us closer to understanding how dementia takes hold and how it might be stopped.

Related: A Single Molecule May Explain How Blood Flow in The Brain Triggers Dementia

Other results from this study may help future research: The dementia burden attributable to insomnia was slightly higher in women than in men, with the greatest number of these cases occurring in people aged 85 and older.

"These findings indicate the need to integrate sleep health into routine geriatric care and dementia prevention strategies," write the researchers.

"Addressing insomnia through targeted, sex-specific interventions could play a crucial role in reducing dementia risk at the population level."

The research has been published in The Journals of Gerontology: Series A.