Six to nine hours of sleep per night is imperative for seniors' cognition, according to researchers at the University of Oregon, who compiled data from aged populations in six middle-income nations.

The results indicate that improving sleep patterns could be critical in preventing and treating dementia.

"We wanted to look at aging, particularly dementia and cognitive decline as people get older, and the importance of sleep. Our results provide compelling evidence that sleep matters a lot," said lead author Theresa E. Gildner, a doctoral student in the UO's anthropology department.

While the importance of sleep for clearer thinking is hardly a new discovery, similar studies in the past have been limited to populations in economic powerhouses such as the United States, Western Europe and Japan.

The study began in 2007 and involved 30,000 participants of at least 50 years of age in China, Ghana, India, Mexico, Russian and South Africa.

"In all six countries, which are very different culturally, economically and environmentally -- despite all these differences -- you see similar patterns emerging," says Gildner.

Findings indicate that women are more likely to struggle with sleep quality, while men reported shorter sleep durations.

The only exceptions were Russia and Mexico, where men reported sleeping longer than women.

Men and women in South Africa sleep the most of the countries concerned, while those in India sleep the least.

While the importance of sleep should never be underestimated, too much of it can also compromise cognition, according to the study.

Individuals who reported sleeping more than nine hours per night had lower cognitive test scores, as did those who sleep less than six hours per night.

Participants were interviewed and tested by trained native speakers in each country. Data was averaged from sleep quality reports, based on a five-point scale, and the number of hours participants had slept in the past two nights.

Next, participants were given standard cognition tests that included but were not limited to immediate recall of a list of words, followed by delayed recall of those words, numerical tests and a verbal fluency test in which they listed as many animals as possible without repetition.

Gildner remarked on the gender differences in the results of the study, which could reflect the hormonal changes of menopause and the stress of losing a spouse.

The first wave of results was published in the June issue of the Journal of Clinical Sleep Medicine, although the investigation is ongoing.