STRIVING for eight hours sleep may be doing you more harm than good, a study has claimed.

It could increase your risk of dementia, a disease that’s diagnosed a massive 10 million times a year worldwide.

Researchers from the Washington University School of Medicine looked at 100 adults in the mid to late 70s.

They tracked them between four and five years, in which time, 12 people showed signs of cognitive decline, including one with mild dementia.

Alzheimer’s is the main cause of cognitive decline in older adults, and is the main form of dementia.

Throughout the study, the participants took part in tests that looked out for signs of cognitive impairment..

Researchers also got an idea of their usual sleep patterns, tracking four to six nights of sleep three years into the study using a EEG machine.

They discovered that, just like too little sleep, too much sleep was a risk factor for an ageing brain.

Getting fewer than 4.5 hours or more than 6.5 hours a night was more commonly seen in those who showed cognitive decline over time.

The effect was similar to age, the biggest driving force of dementia.

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“We know from previous research that lack of sleep is linked to cognitive decline,” Greg Elder, a senior lecturer in psychology and associate director of Northumbria Sleep Research, Northumbria University, commented in the Conversation.

“But it’s less clear why long sleep is linked with cognitive decline. 

“It could be the case that it isn’t necessarily the length of the sleep that matters, but the quality of that sleep when it comes to risk of developing dementia.”

Sleep quality refers to how much time you spend in deep, restorative sleep, which is vital for a number of reasons.

The study showed that having less “slow wave” sleep – the type where you dream and have rapid eye movement (REM) – was most detrimental to the brain’s health.

Co-senior author David Holtzman, MD, a professor of neurology, said the study does suggest "sleep quality may be key, as opposed to simply total sleep".

First author of the study Prof Brendan Lucey said: "Our study suggests that there is a middle range, or ‘sweet spot,’ for total sleep time where cognitive performance was stable over time."

He said people who wake up feeling rested after either little or many hours sleep should not feel compelled to change their habits.

Although the researchers tried to account for other causes of dementia, they would not be able to cover all of them.

For example, there may have been a pre-existing condition that contributed to participant's cognitive decline while causing them to sleep longer each night.

But the study brings into question the golden eight hours – which some experts say is a myth, anyway.

The NHS says “most adults need between six and nine hours of sleep every night” and everyone is different. 

“What matters is that you find out how much sleep you need and then try to achieve it”, it says, adding that if you wake up tired and spend the day longing for a nap, you are not sleeping enough.

Some only need six hours sleep a night and the NHS says a “solid” sleep is what is essential for a “long and healthy life”.

“Regular poor sleep puts you at risk of serious medical conditions, including obesity, coronary heart disease and diabetes – and it shortens your life expectancy,” the NHS warns.

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