Scientists have long said getting a good night's sleep is important to your health .
科学家们早就说过，晚上睡个好觉对健康很重要。
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 .
美国国立卫生研究院说，睡眠不足甚至可能增加大脑活动受损（即认知能力下降）和阿尔茨海默病的风险。
Now , an American team of scientists reports that too much sleep might be similarly linked to such conditions .
现在，一个美国科学家团队报告说，睡得太多可能同样与这些疾病有关。
Researchers at Washington University School of Medicine in St . Louis , Missouri , did the sleep study .
这项睡眠研究由密苏里州圣路易斯的华盛顿大学医学院的研究人员完成。
Their findings were published in the September issue of the scientific publication Brain .
他们的研究结果发表在科学期刊《大脑》九月号上。
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 .
这项研究考察了100名成年人，他们的认知能力（即思考能力）平均接受了4.5年的医学观察。
The average age of the study subject was 75 .
研究对象的平均年龄为75岁。
Eighty-eight of the group had been identified as free of cognitive impairment , or damage .
该组中有88人被确认没有认知障碍或损伤。
Eleven subjects were judged very mildly impaired , and one was identified as mildly impaired .
11人被判定为极轻度受损，1人被确认为轻度受损。
During the study , they were asked to complete several tests for signs of cognitive decline .
研究期间，他们被要求完成几项测试，以检查认知能力下降的迹象。
The tests are combined into a cognitive score – the higher the score , the better .
这些测试的结果合并为一个认知分数——分数越高越好。
They also wore electroencephalography (EEG ) devices for four to six nights to measure their brain activity during sleep .
他们还佩戴了四到六个晚上的脑电图（EEG）设备，以测量睡眠期间的大脑活动。
But the scores stayed the same for those in the middle of the range .
但处于中间范围的人，分数保持不变。
Dr . Brendan Lucey is director of the Washington University Sleep Medicine Center and led the research .
布伦丹·卢西博士是华盛顿大学睡眠医学中心主任，也是这项研究的负责人。
He said , “Our study suggests that there is a middle range , or ‘sweet spot ,’ for total sleep time ” for best cognitive performance .
他说：“我们的研究表明，总睡眠时间存在一个中间范围，或者说‘最佳点’”，可使认知表现达到最佳。
“Short and long sleep times were associated with worse cognitive performance , perhaps due to insufficient sleep or poor sleep quality , ” Lucey added .
“睡眠时间过短和过长都与较差的认知表现有关，这或许是因为睡眠不足或睡眠质量差。”卢西补充说。
Elder also noted that the research did not consider other information about the individuals studied , including their general health or economic conditions .
埃尔德还指出，这项研究没有考虑研究对象的其他信息，包括他们的整体健康状况或经济条件。
Other studies have shown that lack of sleep was linked with cognitive decline .
其他研究表明，睡眠不足与认知能力下降有关。
Beta-amyloid is a metabolic waste product found in between brain cells .
β-淀粉样蛋白是一种存在于脑细胞之间的代谢废物。
It has been long linked to brain disorders , such as Alzheimer's disease .
长期以来，它一直被认为与阿尔茨海默病等脑部疾病有关。
The National Sleep Foundation advises that healthy adults need between seven and nine hours of sleep per night .
美国国家睡眠基金会建议，健康的成年人每晚需要睡七到九个小时。
Babies , young children , and teens need even more sleep for their growth and development .
婴儿、幼儿和青少年为了成长发育，需要更多的睡眠。
And people over 65 should also get seven to eight hours per night , the foundation says .
该基金会还表示，65岁以上的人每晚也应该睡七到八个小时。
One unanswered question from the Washington University study is whether increasing sleep time for short sleepers would help their cognitive performance .
华盛顿大学这项研究尚未解答的一个问题是，增加睡眠少的人的睡眠时间，是否有助于提升他们的认知表现。
But lead writer Lucey said each person's sleep needs are individual .
但主要作者卢西说，每个人对睡眠的需求都不一样。
If people feel rested , there is no need to change how they sleep .
如果人们觉得休息得很好，就没有必要改变睡眠方式。
But those who are not sleeping well should know that sleep problems often can be treated , Lucey said .
但卢西说，睡眠不好的人应该知道，睡眠问题通常是可以治疗的。
Dr . David Holtzman , another top researcher on the study , added , “It suggests that sleep quality may be key , as opposed to simply total sleep ."
这项研究的另一位主要研究人员戴维·霍尔兹曼博士补充说：“这表明，关键可能在于睡眠质量，而不仅仅是总睡眠时间。”