Why Is 7 Hours of Sleep — Not 8 — the Perfect Amount in Middle Age — Scientists Warn Any Less or More Raises Dementia Risk

According to science, eight hours of sleep per day is no longer the ideal length of sleep each night while in middle age.

Researchers at Cambridge University are now recommending that people get seven hours — what they maintain is the sweet spot for warding off dementia and general health. The decline comes after investigating the sleep patterns of half a million individuals in Great Britain ages 38 to 73. 

People who got more or less than seven hours of sleep per night scored more negatively in tests for attention span, thinking speed, problem-solving, and memory.

Until now, getting eight hours of sleep uninterrupted was thought to be in the ‘Goldilocks’ zone for sleep and provides the most advantageous health benefits.

Sleep disruption happens to those who both sleep too long and sleep too little and is associated with plaque build-up in the brain — a tell-tale dementia sign.

Sleep is crucial for brain plasticity — remembering things and processing information—and removing waste products from regulating metabolism, maintaining the immune system, and brain cells.

Britain’s National Health System (NHS) recommends six to nine hours of sleep per night for adults. Children’s developing brains require 12 hours each night.

However, older and middle-aged adults frequently find it more difficult to fall asleep and stay asleep. Experts believe this can trigger psychiatric problems in our senior years and speed up cognitive decline.

Researchers examined data from adults aged 38 to 73 — 498,277 adults — gathered from the UK Biobank — a patient database monitored for ten years.

Participants were queried about their mental health, well-being, and sleeping patterns and took part in a series of cognitive brain tests — completed on a touchscreen tablet — including assessments of their memory and reaction time.

The results, published in the Nature Aging journal, show that excessive and insufficient sleep duration is tied to impaired cognitive performance, including visual attention, problem-solving skills, memory, and processing speed.

Those who received seven hours of sleep nightly had the healthiest brains.

They had the best mental health and cognitive performance — with decreased levels of depression and anxiety and increased well-being — compared to those receiving more or less than seven hours of sleep.

Researchers said one reason for their findings might be that those only getting less than seven hours were suffering disruption to their slow-wave deep sleep. Those individuals who get too much sleep might be affected because lengthy shut-eye can cause fragmented and poor-quality deep sleep.

Sleep interruptions have been linked to a build-up in the brain of amyloid, a protein, which is one of the top theories about how dementia develops.

Amyloid naturally occurs, but increased levels cause the protein to form plaques and clump together, causing ‘tangles’ in the brain that disrupt function.

Researchers noted that a lack of sleep might make it harder for the brain to eliminate toxins.

Brain imaging data from about 40,000 participants showed a tie between the structure of brain regions involved in memory, cognitive processing, and sleep duration.

The team discovered individuals who had less than seven hours and more substantial changes in these areas — including the lateral orbitofrontal cortex, precentral cortex, and hippocampus.

Researchers additionally noted that getting seven hours of sleep consistently nightly ‘without too much fluctuation in duration’ was critical for mental health and brain function.

Co-author of the study and neuroscientist at the university, Professor Barbara Sahakian, said: “Getting a good night’s sleep is important at all stages of life, but particularly as we age. 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.”

Professor Jianfeng Feng, brain expert at China’s Fudan University and co-author of the study, said: “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.”

“But the reasons why older people have poorer sleep appear to be complex, influenced by a combination of our genetic makeup and the structure of our brains,” concluded Feng.