Sleeping for more than nine hours a night could be an early warning sign of Alzheimer's disease.
Scientists found people who consistently spend this long in bed are twice as likely to develop dementia over the next decade.
A change in sleep patterns is a red flag for Alzheimer's as it shows the brain, which controls wakefulness, has suffered damage, reports DailyMail.
Researchers also found those who slept nine hours or longer also had smaller brain volumes, took longer to process information and showed signs of memory loss.
Crucially, an inability to get out of bed is believed to be a symptom rather than a cause of the brain changes that lead to Alzheimer's.
It means older people cannot ward off the condition by setting their alarm clock earlier.
But the study of more than 2,400 people, who were followed up for 10 years, provides a new insight into dementia.
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."
The study comes after research this week suggested rambling, long-winded speech could be a warning sign for Alzheimer's.
Another is when someone begins to lose their sense of smell, with experts using a so-called "peanut butter test" to test if people can sniff out the spread from a distance.
The latest research looked at participants with an average age of 72, enrolled in the Framingham Heart Study, a major US investigation into heart disease risk factors.
The participants were asked how long they typically slept each night and followed up over a decade, with 234 cases of dementia 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.
Participants without a high school degree who slept for more than nine hours increased their risk six-fold, suggesting that education may protect against the condition.
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.
"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."
However she called for further research, as the results were based on self-reported sleep information.
Previous studies have shown people with early dementia also suffer from interrupted sleep, thought to be a similar sign of neurodegeneration.
But sleep can also ward off dementia, with too little suggested by some experts to be a risk factor for the condition.
The US study, published in the journal Neurology, states: "Sleep may provide a restorative function, removing metabolic waste from the brain and preventing the accumulation of (protein) B-amyloid, a hallmark of Alzheimer disease.
"On the other hand, sleep disorders may also emerge as a result of atrophy to brain regions involved in sleep and wakefulness, or as a consequence of mood disturbances, which are common in dementia."