An hour-long nap after lunch may help older adults to preserve their memories, improve their ability to think clearly as well as to make decisions, a study has found.Sleep plays a key role in helping older adults maintain their healthy mental function, necessary for people as they age, the researchers said.In the study, led by Junxin Li from the University of Pennsylvania, Philadelphia, the team examined nearly 3,000 Chinese adults aged 65 and older to learn whether taking an afternoon nap had any effect on their mental health.The researchers found that nearly 60 per cent of the people took an afternoon nap after lunch.Their nap time was between about 30 minutes to more than 90 minutes, with most people taking naps lasting about 63 minutes.The results showed that people who took an hour-long nap after lunch had better health condition compared to people who did not take a nap -- neither shorter nor longer. Conversely, those who took no naps at all had four-to-six times more decrease in their mental ability. In addition, people who did not take a nap at all, and those who took shorter or longer naps, experienced about the same decline in their mental abilities that a five-year increase in age would be expected to cause, Li stated. The study was published in the Journal of the American Geriatrics Society.