Scientists have long said getting a good night's sleep is important to your health .
科學家們早就說過，晚上睡個好覺對健康很重要。
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 .
美國國家衛生研究院說，睡眠不足甚至可能增加大腦活動受損（即認知能力下降）和阿茲海默症的風險。
Now , an American team of scientists reports that too much sleep might be similarly linked to such conditions .
現在，一個美國科學家團隊報告說，睡得太多可能同樣與這些疾病有關。
Researchers at Washington University School of Medicine in St . Louis , Missouri , did the sleep study .
這項睡眠研究由密蘇里州聖路易的華盛頓大學醫學院的研究人員完成。
Their findings were published in the September issue of the scientific publication Brain .
他們的研究結果發表在科學期刊《大腦》九月號上。
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 .
這項研究調查了100名成年人，他們的認知能力（即思考能力）平均接受了4.5年的醫學觀察。
The average age of the study subject was 75 .
研究對象的平均年齡為75歲。
Eighty-eight of the group had been identified as free of cognitive impairment , or damage .
該組中有88人被確認沒有認知障礙或損傷。
Eleven subjects were judged very mildly impaired , and one was identified as mildly impaired .
11人被判定為極輕度受損，1人被確認為輕度受損。
During the study , they were asked to complete several tests for signs of cognitive decline .
研究期間，他們被要求完成幾項測試，以檢查認知能力下降的跡象。
The tests are combined into a cognitive score – the higher the score , the better .
這些測試的結果合併為一個認知分數——分數越高越好。
They also wore electroencephalography (EEG ) devices for four to six nights to measure their brain activity during sleep .
他們還配戴了四到六個晚上的腦電圖（EEG）設備，以測量睡眠期間的大腦活動。
But the scores stayed the same for those in the middle of the range .
但處於中間範圍的人，分數保持不變。
Dr . Brendan Lucey is director of the Washington University Sleep Medicine Center and led the research .
布倫丹·盧西博士是華盛頓大學睡眠醫學中心主任，也是這項研究的負責人。
He said , “Our study suggests that there is a middle range , or ‘sweet spot ,’ for total sleep time ” for best cognitive performance .
他說：「我們的研究顯示，總睡眠時間存在一個中間範圍，或者說『最佳點』」，可使認知表現達到最佳。
“Short and long sleep times were associated with worse cognitive performance , perhaps due to insufficient sleep or poor sleep quality , ” Lucey added .
「睡眠時間過短和過長都與較差的認知表現有關，這或許是因為睡眠不足或睡眠品質差。」盧西補充說。
Elder also noted that the research did not consider other information about the individuals studied , including their general health or economic conditions .
埃爾德還指出，這項研究沒有考慮研究對象的其他資訊，包括他們的整體健康狀況或經濟條件。
Other studies have shown that lack of sleep was linked with cognitive decline .
其他研究顯示，睡眠不足與認知能力下降有關。
Beta-amyloid is a metabolic waste product found in between brain cells .
β-類澱粉蛋白是一種存在於腦細胞之間的代謝廢物。
It has been long linked to brain disorders , such as Alzheimer's disease .
長期以來，它一直被認為與阿茲海默症等腦部疾病有關。
The National Sleep Foundation advises that healthy adults need between seven and nine hours of sleep per night .
美國國家睡眠基金會建議，健康的成年人每晚需要睡七到九個小時。
Babies , young children , and teens need even more sleep for their growth and development .
嬰兒、幼兒和青少年為了成長發育，需要更多的睡眠。
And people over 65 should also get seven to eight hours per night , the foundation says .
該基金會還表示，65歲以上的人每晚也應該睡七到八個小時。
One unanswered question from the Washington University study is whether increasing sleep time for short sleepers would help their cognitive performance .
華盛頓大學這項研究尚未解答的一個問題是，增加睡眠少的人的睡眠時間，是否有助於提升他們的認知表現。
But lead writer Lucey said each person's sleep needs are individual .
但主要作者盧西說，每個人對睡眠的需求都不一樣。
If people feel rested , there is no need to change how they sleep .
如果人們覺得休息得很好，就沒有必要改變睡眠方式。
But those who are not sleeping well should know that sleep problems often can be treated , Lucey said .
但盧西說，睡眠不好的人應該知道，睡眠問題通常是可以治療的。
Dr . David Holtzman , another top researcher on the study , added , “It suggests that sleep quality may be key , as opposed to simply total sleep ."
這項研究的另一位主要研究人員大衛·霍爾茲曼博士補充說：「這顯示，關鍵可能在於睡眠品質，而不僅僅是總睡眠時間。」