How could something that feels so good - a long night's sleep - have negative consequences? Unfortunately, that is one possibility that results of a new study suggest: Older adults who sleep nine or more hours each day may have a higher risk of developing dementia than those who spend fewer hours in bed.

Spanish researchers found that among nearly 3,300 older adults they followed for three years, those who slept nine or more hours per day, daytime naps included, were about twice as likely to develop dementia as those who typically slept for seven hours.

These long sleepers were at increased risk even when the researchers accounted for several factors that can affect both sleep and dementia risk -- including age, education, and smoking and drinking habits.

Still, the findings show only an association between longer sleep and dementia, and do not prove that extra hours in bed, per se, contribute to mental decline.

It remains to be established how the relation between longer sleep duration and dementia is mediated, Dr. Julian Benito-Leon, of University Hospital '12 de Octubre' in Madrid, told Reuters Health in an email.

One possibility, according to Benito-Leon, is that increased fatigue and sleep is an initial sign of early dementia in some people.

Another theory is that one or more underlying health problems may increase older adults' need for sleep, as well as contribute to dementia. The breathing disorder sleep apnea, for instance, causes fatigue and has been linked to impairments in thinking and memory in older adults.

It's also possible, Benito-Leon said, that excessive sleep somehow directly affects dementia risk -- though, if that is true, there is no known physiological explanation.

The study findings, which appear in the European Journal of Neurology, are based on evaluations of 3,286 adults age 65 and older. At the outset, all were screened for dementia and reported on their typical sleep habits.

Over the next three years, 140 study participants were diagnosed with dementia. Among those who had said they slept at least nine hours per day, just over 5 percent developed dementia. That compared with roughly 2 percent of men and women who slept for seven hours per day, and 4 percent of those who logged eight hours.

On the other end of the spectrum, 5 percent of short sleepers -- those who got five or fewer hours of sleep per day -- were diagnosed with dementia during the study. However, when the researchers weighed other factors, lack of sleep, in and of itself, was no longer linked to a higher dementia risk.

While excessive sleep may or may not contribute to dementia, it could be considered a potential sign of a problem, according to Benito-Leon.

He recommended that if an older adult who has typically slept the standard seven or eight hours per day suddenly starts needing more sleep, he or she should discuss the change with a doctor.

SOURCE: European Journal of Neurology, September 2009.