Don’t feel guilty about getting a full night’s sleep: New research suggests that people who sleep less than six hours on average in middle age are 30 percent more likely to develop dementia when they are older. The New York Times reports that the study published in Nature is significant because the subjects were followed for decades—making it likely that their shortened sleep habits were not the result of changes in the brain that ultimately lead to dementia.

“I don’t know that this study necessarily seals the deal, but it gets closer because it has a lot of people who were relatively young,” Dr. Erik Musiek, a neurologist not involved in the study, told the paper. “There’s a decent chance that they are capturing people in middle age before they have Alzheimer’s disease pathology or plaques and tangles in their brain.” Those who slept a modest seven hours were at lower risk, the study found. What scientists haven’t figured out is exactly why getting less might lay the groundwork for dementia.