Napping frequently during daytime could be an early sign of dementia, or may even prompt it -- so beware, say neurologists.

They have linked excessive sleep and problems with thinking in the elderly.

Many older people take 40 winks, and it is usually only because they need physical rest from exertion.

But French researchers who looked at results from 5,000 people over 65 years found the fifth who regularly took long naps scored lower in mental ability tests, according to the Telegraph.

Claudine Berr, from the Institut National de la Sante et de la Recherche Medical (Inserm), told the Alzheimer's Association International Conference in Vancouver: "These results suggest that excessive daytime sleepiness may be an early predictor of cognitive decline."

In another study, also presented in Vancouver, American researchers found that routinely sleeping more than nine hours a night, or less than five, was linked to lower mental ability.

Elizabeth Devore from the Brigham and Women's Hospital in Boston, Massachusetts, looked at 15,000 former nurses over 70.

She and colleagues also found those who slept too much or too little exhibited chemical brain changes indicative of early Alzheimer's, which is the commonest form of dementia.

She said that over time, "extreme sleep durations... may contribute to cognitive decline and early Alzheimer's", and not simply be a passive sign of them.

Marie Janson of Alzheimer's Research in Britain said: "There is already some evidence linking sleep duration and disturbances to cardiovascular health and diabetes. So it is not surprising to see studies examining how sleep might affect cognitive ability over time."

*Text courtesy IANS