Seven hours of sleep each night is the ideal amount between middle age and old age, according to research.
The study of almost 500,000 adults aged 38 to 73 found that too much and too little sleep were linked to poorer cognitive performance and mental health, including anxiety and depression. A consistent amount of sleep also seemed to be beneficial.
Professor Barbara Sahakian, from Cambridge University’s Department of Psychiatry, said: ‘For every hour you’ve been seven hours away, your condition has gotten worse. It is very clear that the processes that take place in our brain during sleep are very important for maintaining our physical and mental health.
Getting a good night’s sleep, she added, is important at all stages of life, but especially as people age. “I think it’s as important as exercising.”
One possible reason for the link between lack of sleep and cognitive decline may be disruption of slow or deep sleep, which has been shown to be important for memory consolidation. A lack of deep sleep could also prevent the brain from efficiently eliminating toxins.
The study, which used data from the UK Biobank, included brain imaging and genetic data for nearly 40,000 of the study participants. She found that the brain area most affected by sleep was the region containing the hippocampus, the brain’s memory center, with too much or too little sleep linked to smaller brain volume. The analysis found that people who slept seven hours a night performed better on average on cognitive tests of processing speed, visual attention, memory and problem solving.
However, the work could not prove a causal relationship, and the link between sleep and certain brain disorders is complex. For example, unusual sleep patterns and insomnia are a common symptom in people with dementia.
Scientists are less clear on why spending eight or more hours in bed could cause problems. One explanation is that people with poor quality and disrupted sleep tend to sleep longer – or try to sleep – because they feel tired. “We don’t really understand why sleeping longer would be a problem,” Sahakian said.
Professor Jianfeng Feng, from Fudan University in Shanghai, said: “While we cannot say conclusively that too little or too much sleep causes cognitive problems, our analysis looking at individuals over a longer period seems to support this idea.
“The reasons why older people sleep less appear to be complex, influenced by a combination of our genetic make-up and brain structure.”
The research is published in the journal Nature Aging.