Older adults who started sleeping more than nine hours a night — but had not previously slept so much — were at more than double the risk of developing dementia a decade later than those who slept nine hours or less, researchers report.

The increased risk was not seen in people who had always slept more than nine hours.

“We’re not suggesting you go wake up Grandpa. We think this might be a marker for the risk of dementia, not a cause” of the illness, said Dr. Sudha Seshadri, a professor of neurology at Boston University School of Medicine and the senior author of the study, in Neurology.