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The optimal amount of sleep is not too little but not too much – at least in middle age and old age.
New research has shown that around seven hours of sleep is the ideal night’s rest, with both insufficient and excessive sleep associated with a reduced ability to pay attention, remember and learn new things, solve problems and to take decisions.
Seven hours of sleep was also found to be linked to better mental health, with people experiencing more symptoms of anxiety and depression, and worse general well-being if they reported sleeping for longer or shorter periods.
“Although we can’t say conclusively that too little or too much sleep causes cognitive problems, our analysis of individuals over a longer period of time seems to support this idea,” said Jianfeng Feng, a professor at the University. Fudan from China and author of the study published in the scientific journal Nature Aging, said in a statement.
“But the reasons why older people sleep less appear to be complex, influenced by a combination of our genetic make-up and brain structure.”
Researchers from China and the UK analyzed data from nearly 500,000 adults aged 38 to 73 who were part of the UK Biobank – a government-backed long-term health study. Participants were asked about their sleep habits, mental health and well-being, and took part in a series of cognitive tests. Brain imaging and genetic data were available for almost 40,000 of the study participants.
Other research has shown that older people who have significant difficulty falling asleep and who wake up frequently at night are at high risk of developing dementia or dying early from any cause, while sleeping less than six hours a night has been linked to cardiovascular disease.
One reason for the link between lack of sleep and cognitive decline could be the disruption of deep sleep, which is when the brain repairs the body from the wear and tear of the day and consolidates memories. Too little sleep is also associated with the buildup of amyloid, a key protein that can cause tangles in the brain that characterize some forms of dementia. The study also indicated that it is possible that prolonged sleep duration stems from poor quality fragmented sleep.
Dr. Raj Dasgupta, spokesperson for the American Academy of Sleep Medicine and assistant professor of clinical medicine at the University of Southern California’s Keck School of Medicine, said longer sleep durations have been associated with cognitive issues, but it wasn’t entirely clear. Why.
“It sets a mark for future research and finding a cure,” said Dasgupta, who was not involved in the research. “Sleep is essential as we age, and we need it as much as young people, but it’s harder to find.”
The study had some limitations – it only assessed participants’ total sleep time and no other measures of sleep quality, such as waking up during the night. Additionally, participants reported how much they slept, so it was not objectively measured. However, the authors said the large number of people involved in the study meant its findings were likely strong.
The authors said their findings suggested it was important that sleep, ideally around seven hours, was consistent.
The study showed a link between too much and too little sleep and cognitive problems, not cause and effect, warned Russell Foster, a professor at Oxford University and director of the Sir Jules Thorn Sleep and Circadian Neuroscience Institute, who was not involved in the research. He said the study did not take into account the health status of individuals and that short or long sleep could be an indication of underlying health conditions with cognitive problems.
He also said that taking the average of seven hours as the ideal length of sleep “ignores the fact that there is considerable individual variation in sleep duration and quality.” Less or more sleep can be perfectly healthy for some people, he said.
“We are regularly told that the ‘ideal’ night’s sleep for older adults should be seven hours of uninterrupted sleep. This belief is wrong in many ways. Sleep is like shoe size; one size does not fit all, and classifying ‘good sleep’ in this way may cause confusion and anxiety for many,” said Foster, author of the forthcoming book “Life Time: The New Science of the Body Clock, and How It Can”. Revolutionize your sleep and your health.
“How long we sleep, our preferred sleep times and the number of times we wake up during the night vary enormously from individual to individual and as we age. Sleep is dynamic, and we all have different sleep patterns, and the key is to assess what our individual needs are.