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 cognitive(认知) 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 did the sleep study. 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 damage. Twelve subjects were judged very mildly damaged.
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.
Overall, cognitive scores declined for those who slept less than 4.5 hours or more than 6.5 hours per night -as measured by EEG(脑电图). But the scores stayed the same for those in the middle of the range.
Dr. Brendan Lacey is the director of the Washington University Sleep Medicine Center and led the research. He said, "Our study suggests that there is'a sweet spot' for total sleep time for best cognitive performance. Short and long sleep times are associated with worse cognitive performance, perhaps due to insufficient sleep or poor sleep quality," Lacey added.
Greg Elder is a sleep researcher at Northumbria University in New Castle, Britain. He was surprised about the findings that sleeping longer than 6.5 hours was linked with cognitive decline. Elder wrote in The Conversation, "Each person's sleep needs are individual. If people feel rested, there is no need to change how they sleep." He also noted that it suggests that sleep quality may be the key, as opposed to simply total sleep time.
