Scientists have long said getting a good night's sleep is important to your health .
The U.S . National Institute of Health says lack of sleep may even increase the risk of impaired brain activity , or cognitive decline , and Alzheimer's disease .
Now , an American team of scientists reports that too much sleep might be similarly linked to such conditions .
Researchers at Washington University School of Medicine in St . Louis , Missouri , did the sleep study .
Their findings were published in the September issue of the scientific publication Brain .
The study examined 100 adults who had been under medical observation for cognitive ability , the ability to think , for an average of 4.5 years .
The average age of the study subject was 75 .
Eighty-eight of the group had been identified as free of cognitive impairment , or damage .
Eleven subjects were judged very mildly impaired , and one was identified as mildly impaired .
During the study , they were asked to complete several tests for signs of cognitive decline .
The tests are combined into a cognitive score – the higher the score , the better .
They also wore electroencephalography (EEG ) devices for four to six nights to measure their brain activity during sleep .
But the scores stayed the same for those in the middle of the range .
Dr . Brendan Lucey is director of the Washington University Sleep Medicine Center and led the research .
He said , “Our study suggests that there is a middle range , or ‘sweet spot ,’ for total sleep time ” for best cognitive performance .
“Short and long sleep times were associated with worse cognitive performance , perhaps due to insufficient sleep or poor sleep quality , ” Lucey added .
Elder also noted that the research did not consider other information about the individuals studied , including their general health or economic conditions .
Other studies have shown that lack of sleep was linked with cognitive decline .
Beta-amyloid is a metabolic waste product found in between brain cells .
It has been long linked to brain disorders , such as Alzheimer's disease .
The National Sleep Foundation advises that healthy adults need between seven and nine hours of sleep per night .
Babies , young children , and teens need even more sleep for their growth and development .
And people over 65 should also get seven to eight hours per night , the foundation says .
One unanswered question from the Washington University study is whether increasing sleep time for short sleepers would help their cognitive performance .
But lead writer Lucey said each person's sleep needs are individual .
If people feel rested , there is no need to change how they sleep .
But those who are not sleeping well should know that sleep problems often can be treated , Lucey said .
Dr . David Holtzman , another top researcher on the study , added , “It suggests that sleep quality may be key , as opposed to simply total sleep ."