Sleeping too long at night and daytime sleepiness could be an early sign of dementia, French researchers have warned.
Sleep quality and quantity has long been associated with ageing, so Dr Claudine Berr from France's National Institute of Health and Medical Research (INSERM) led a team who analysed data from almost 4900 people aged over 65 years who had completed sleep questionnaires and undergone mental examinations.
They measured the participants after two, four and eight years, looking for signs of insomnia: poor sleep quality, difficulty initiating sleep, difficulty maintaining sleep, waking early and excessive daytime sleepiness.
They found the 17.9 percent who experienced excessive daytime sleepiness, had an increased risk of cognitive decline.
But difficulty maintaining sleep did not appear to be linked with cognitive decline.
"These results suggest that excessive daytime sleepiness may be an early predictor of cognitive decline and that sleep complaints should be adequately evaluated in older persons," the researchers told the Alzheimer's Association International Conference in Vancouver.
In a separate US study, researchers found people over the age of 70 who slept less than five hours a day or more than nine hours per day had reduced mental capacity.
In fact, their brains had aged the cognitive equivalent of two years, compared to those who clocked seven hours' sleep each night.
The researchers also found those who slept too little or too much had brain changes that point to early Alzheimer's.
Elizabeth Devore from the Brigham and Women's Hospital in Boston, Massachusetts, led the research team who studied the data of more than 15,000 women.
"Our findings support the notion that extreme sleep durations and changes in sleep duration over time may contribute to cognitive decline and early Alzheimer's changes in older adults," Devore said.
"The public health implications of these findings could be substantial, as they might lead to the eventual identification of sleep and circadian-based strategies for reducing risk of cognitive impairment and Alzheimer's."
Dr William Thies, the Alzheimer's Association chief medical and scientific officer, said experts now know people with sleep problems could experience cognitive decline.
"The good news is that tools already exist to monitor sleep duration and quality and to intervene to help return sleep patterns to normal," Dr Thies said.
"If we do this, there is the possibility that we may also help people preserve their cognitive health, but that needs to be tested."