Seven hours of sleep per night is the ideal amount in middle to older age, a UK study has suggested.
Both too much and too little sleep are associated with poorer cognitive performance and mental health, according to researchers from Cambridge University and Fudan University in China.
Scientists examined data from nearly 500,000 adults, aged between 38 and 73, from the UK Biobank.
Participants were asked about their sleeping patterns, 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.
The researchers’s analysis of the data indicated that seven hours of sleep per night was the optimal duration for cognitive performance, such as processing speed, visual attention, memory and problem-solving skills.
Prof Barbara Sahakian, Cambridge University’s Department of Psychiatry, said her team's research showed that seven hours of sleep was superior to the typical recommendation of eight hours, particularly for those above 40.
"Seven-hour sleep duration on a regular basis is especially beneficial for the brain, cognition and mental health measures for the age group 44 to 59 years," she told The National.
"This is important because adults of working age benefit most from consistently getting seven hours of sleep"
The findings are in contrast to advice from the UK's National Health Service, which recommends between seven to nine hours of sleep for adults.
But the relationship may be different for younger people and adolescents whose brains are still developing, Prof Sahakian said.
It was also optimal for good mental health, with people experiencing more symptoms of anxiety and depression, and worse overall well-being, if they reported sleeping for longer or shorter hours.
The researchers say one possible reason for the association between insufficient sleep and cognitive decline may be due to the disruption of slow-wave – “deep” – sleep.
Disruption to this type of sleep has been shown to have a close link with memory consolidation as well as the build-up of amyloid – a key protein which, when it misfolds, can cause “tangles” in the brain characteristic of some forms of dementia.
Additionally, lack of sleep may hamper the brain’s ability to rid itself of toxins.
Prof Jianfeng Feng, from Fudan University, said the study pointed to a possible link between sleep and cognitive performance.
“While we can’t say conclusively that too little or too much sleep causes cognitive problems, our analysis looking at individuals over a longer period of time appears to support this idea," he said.
“The reasons why older people have poorer sleep appear to be complex, influenced by a combination of our genetic make-up and the structure of our brains.”
The researchers say the findings suggest that insufficient or excessive sleep duration may be a risk factor for cognitive decline in ageing.
“Getting a good night’s sleep is important at all stages of life, but particularly as we age", Prof Sahakian said.
“Finding ways to improve sleep for older people could be crucial to helping them maintain good mental health and well-being, and avoiding cognitive decline, particularly for patients with psychiatric disorders and dementias.”
The research is published in the journal Nature Aging.