Study Suggests Link Between Long Sleep Times, Cognitive Loss

Scientists have long maintained that a good night’s sleep is essential to health. According to the U.S. National Institute of Health, insufficient sleep may even raise the risk of impaired brain function, known as cognitive decline, as well as of Alzheimer’s disease. Now, however, a team of American researchers reports that sleeping too much may be linked to the same conditions.

The study, carried out at Washington University School of Medicine in St. Louis, Missouri, appeared in the September issue of the scientific journal Brain. It examined 100 adults whose cognitive ability, that is, their ability to think, had been under medical observation for an average of four and a half years. The participants were 75 years old on average. Eighty-eight of them had been judged free of cognitive impairment, eleven were considered very mildly impaired, and one mildly impaired.

Over the course of the study, participants completed a series of tests designed to detect signs of cognitive decline, the results of which were combined into a single cognitive score, with higher scores indicating better performance. In addition, they wore electroencephalography (EEG) devices for four to six nights so that their brain activity during sleep could be measured.

Overall, cognitive scores fell among those whose EEG results showed that they slept for less than 4.5 hours or more than 6.5 hours a night, whereas they remained stable for those in between. Dr. Brendan Lucey, director of the Washington University Sleep Medicine Center, who led the research, concluded that there appears to be a middle range, or “sweet spot,” of total sleep time for the best cognitive performance. Both short and long sleep, he added, were associated with poorer performance, possibly because of insufficient sleep or poor sleep quality.

Not everyone found the results easy to accept. Greg Elder, a sleep researcher at Northumbria University in Newcastle, Britain, was surprised that sleeping more than 6.5 hours should be linked with cognitive decline. Writing in The Conversation, he observed that this figure is low, given that older adults are advised to sleep between seven and eight hours a night. He also noted that the researchers had not taken into account other information about the participants, such as their general health or economic circumstances.

The link between too little sleep and cognitive decline, by contrast, is supported by earlier work. A small 2018 study published in the Proceedings of the National Academy of Sciences found that losing a single night’s sleep led to an increase in the production of beta-amyloid, a metabolic waste product found between brain cells that has long been associated with brain disorders such as Alzheimer’s disease.

The National Sleep Foundation recommends that healthy adults sleep for between seven and nine hours a night. Babies, young children and teenagers need even more sleep for their growth and development, while people over 65, the foundation says, should also aim for seven to eight hours.

One question the Washington University study leaves open is whether sleeping longer would improve the cognitive performance of people who sleep too little. Lucey stresses that sleep needs vary from person to person: those who feel rested have no reason to change their habits, but those who are not sleeping well should be aware that sleep problems can often be treated. As Dr. David Holtzman, another senior researcher on the study, put it, the findings suggest “that sleep quality may be key, as opposed to simply total sleep.”