The correlation also held whether or not people were taking sleep medication and whether or not they had a mutation called ApoE4 that makes people more likely to develop Alzheimer’s, Dr. Sabia said.
The researchers found no general difference between men and women.
“The study found a modest, but I would say somewhat important association of short sleep and dementia risk,” said Pamela Lutsey, an associate professor of epidemiology and community health at the University of Minnesota, who was not involved in the research. “Short sleep is very common and because of that, even if it’s modestly associated with dementia risk, it can be important at a societal level. Short sleep is something that we have control over, something that you can change.”
Still, as with other research in this area, the study had limitations that prevent it from proving that inadequate sleep can help cause dementia. Most of the sleep data was self-reported, a subjective measure that isn’t always accurate, experts said.
At one point, nearly 4,000 participants did have sleep duration measured by accelerometers and that data was consistent with their self-reported sleep times, the researchers said. Still, that quantitative measure came late in the study, when participants were about 69, making it less useful than if it had been obtained at younger ages.
In addition, most participants were white and better educated and healthier than the overall British population. And in relying on electronic medical records for dementia diagnoses, researchers might have missed some cases. They also could not identify exact types of dementia.
“It’s always difficult to know what to conclude from these kinds of studies,” wrote Robert Howard, a professor of old age psychiatry at University College London, one of several experts who submitted comments about the study to Nature Communications. “Insomniacs — who probably don’t need something else to ruminate about in bed,” he added, “shouldn’t worry that they are heading for dementia unless they get off to sleep immediately.”
There are compelling scientific theories about why too little sleep might exacerbate the risk of dementia, especially Alzheimer’s. Studies have found that cerebrospinal fluid levels of amyloid, a protein that clumps into plaques in Alzheimer’s, “go up if you sleep-deprive people,” Dr. Musiek said. Other studies of amyloid and another Alzheimer’s protein, tau, suggest that “sleep is important for clearing proteins from the brain or limiting the production,” he said.