Inability to get out of bed may be a symptom of the devastating disease.
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. 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.
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 Centre, 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.'