The findings, which appear in the journal Neurology, found that those who slept for longer also had smaller brain volumes.
Researchers said excessive sleep may be an early warning sign, rather than a cause, of the brain changes that occur with dementia.
They said the findings could help predict those who are at risk from developing dementia.
There are 850,000 people with dementia in the UK, according to the Alzheimer’s Society, and numbers are set to rise to over one million by 2025.
For the new study, adults enrolled in the Framingham Heart Study (FHS) were asked to indicate how long they typically slept each night and were then observed for 10 years to determine who developed dementia.
Researchers from Boston University School of Medicine (BUSM) then analysed the sleep duration data and examined the risk of developing dementia.
“Participants without a high school degree who sleep for more than nine hours each night had six times the risk of developing dementia in 10 years as compared to participants who slept for less,” said co-author Sudha Seshadri, professor of neurology at BUSM.
Researchers said the results suggest that excessive sleep may be a symptom rather than a cause of the brain changes that occur with dementia.
Therefore, interventions to restrict sleep duration are unlikely to reduce the risk of dementia.
“Self-reported sleep duration may be a useful clinical tool to help predict persons at risk of progressing to clinical dementia within 10 years,” said co-author Matthew Pase, fellow in the department of neurology at BUSM.
“Persons reporting long sleep time may warrant assessment and monitoring for problems with thinking and memory,”
The researchers believe screening for sleeping problems may aid in the early detection of cognitive impairment and dementia.
The early diagnosis of dementia has many important benefits, such as providing a patient the opportunity to more activity direct their future plans and health care decisions.