Those who slept for nine hours or longer also had brains with smaller volumes, took longer to process information and showed signs of memory loss, the US researchers found.

London - Sleeping for more than nine hours a night could be an early warning sign of Alzheimer’s disease.

Scientists found that people who consistently spend this long in bed were twice as likely to develop dementia over the next decade.

A change in sleep patterns is a red flag for Alzheimer’s as it could suggest the brain has suffered damage.

Those who slept for nine hours or longer also had brains with smaller volumes, took longer to process information and showed signs of memory loss, the US researchers found.

The study 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, with 234 cases of dementia recorded over the follow-up period.

Sleeping for more than nine hours more than doubled the risk of all types of dementia and Alzheimer’s specifically. The study followed more than 2 400 people for ten years.

Crucially, an inability to get out of bed was believed to be a symptom rather than a cause of the brain changes that lead to Alzheimer’s – meaning older people cannot ward off the condition by setting their alarm clock earlier.

Lead author Dr Matthew Pase, from Boston University Medical Centre, said: "Self-reported sleep duration may be a useful clinical tool to help predict if a person is at risk of progressing to clinical dementia within ten years.

"Persons reporting long sleep-time may warrant assessment and monitoring for problems with thinking and memory."

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 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."

The study, published in the journal Neurology, states: "Sleep disorders may 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." 

It comes after research this week suggested rambling, long-winded speech could be a warning sign for Alzheimer’s.

Another sign is when someone begins to lose their sense of smell, with experts using a so-called "peanut butter test" to see if people can sniff out the spread from a distance.

Previous studies have shown people with early dementia also suffer from interrupted sleep, thought to be a similar sign of neurodegeneration.