Sleeping for more than nine hours a night could be an early warning sign of Alzheimer's, new research suggests.
Scientists believe an inability to get out of bed may be a symptom rather than a cause of the brain changes that lead to dementia.
For this reason, simply reducing the length of sleep time is unlikely to reduce a person's risk.
Researchers in the US found that people who consistently slept for more than nine hours each night were twice as likely to develop Alzheimer's or other forms of dementia over the next 10 years as those sleeping nine hours or less.
Participants without a high school degree who slept for more than nine hours increased their risk six-fold, suggesting that education lessened the effect.
Lead author Dr Matthew Pase, from Boston University Medical Center, said: "Self-reported sleep duration may be a useful clinical tool to help predict persons at risk of progressing to clinical dementia within 10 years.
"Persons reporting long sleep time may warrant assessment and monitoring for problems with thinking and memory."
Another study has found that developing rambling speech may be an early indication of mild cognitive impairment (MCI), which can lead to Alzheimer's.
An experiment in which people were asked to create a sentence out of three words was described at the American Association for the Advancement of Science (AAAS) annual meeting in Boston.
Lead researcher Dr Janet Cohen Sherman, from Massachusetts General Hospital, said: "The MCIs are very long-winded."
The new findings are based on data from more than 2,400 patients enrolled in the Framingham Heart Study, a major US investigation into heart disease risk factors.
Participants, who had an average age of 72, were asked how long they typically slept each night and observed over a period of 10 years.
A total of 234 cases of dementia were recorded over the follow-up period.
Sleeping for more than nine hours more than doubled the risk of both all types of dementia and specifically Alzheimer's. It was also associated with smaller brain volume.
Dr Rosa Sancho, from Alzheimer's Research UK, said: "While unusual sleep patterns are common for people with dementia, this study adds to existing research suggesting that changes in sleep could be apparent long before symptoms like memory loss start to show.
"Other studies have indicated a link between changes in sleep quality and the onset of dementia, and while this wasn't measured in this study, it could be an important factor affecting sleep duration.
"Understanding more about how sleep is affected by dementia could one day help doctors to identify those who are at risk of developing the condition. This study used self-reported sleep information, which is not always reliable, so larger studies looking at a number of sleep-related factors will be needed to better understand this link."
The research is reported in the journal Neurology.