It’s well known that not getting enough sleep can increase dementia risk, but scientists have now discovered that too much shut eye could be just as harmful.

Researchers have found that sleeping an average of nine hours a night can fuel brain ageing that lead to memory problems in later life.

The study, by scientists at the University of Texas Health Science Centre, found that this amount of sleep added an average of six and a half years to the age of participants’ brains. 

Commenting on the findings they concluded that ‘long sleep duration’ could be ‘an early and potentially reversible’ risk factor for the memory-robbing disease. 

The study followed 1,853 healthy adults aged 27-85, and studied how their sleep duration affected cognitive abilities.

The participants’ cognitive function was assessed every four years using gold standard measurements of memory, verbal reasoning, visual-spatial awareness and reaction times.

Participants also completed a survey every four years, detailing the number of hours they typically slept each night.

Results showed that, on average, those who slept for nine or more hours per night over the course of the two decade study performed significantly worse in all four cognitive tests.

Disturbances in sleep duration have been linked to an increased risk of cognitive defects and Alzheimer’s

Alzheimer’s disease is the most common cause of dementia. The disease can cause anxiety, confusion and short-term memory loss

Scientists found the poorest results were seen in people who showed depressive symptoms, and had nine or more hours sleep a night, on average.

The mood disorder is known to trigger patterns of excessive sleep, and the experts pose that depression, rather than sleep itself, may be driving the cognitive decline in these patients. 

However, its not just those who suffer from low mood that are at risk from too much rest.  

Participants who showed no signs of depression, but slept for more than nine hours a night, experienced decreased cognitive function, struggling with global cognition and visual reproduction. 

‘Disturbances in sleep duration and patterns contribute to an increased risk of cognitive deficits and Alzheimer’s disease’, Prof Young explained. 

The most recent findings echo that of previous studies, including one 10-year-long report, published in the Psychiatry Research journal, that found sleeping for more than eight hours a night, on average, was linked with an alarming 64 per cent increased risk of dementia.

Around 900,000 Britons are currently thought to have the memory-robbing disorder

University College London scientists estimate this will rise to 1.7 million within two decades as people live longer

Whilst experts are not entirely sure why excess shut eye could contribute to dementia, one Swedish study suggested the explanation may lie with the impact on our circadian rhythms—the natural sleep-wake cycles that determine a host of bodily functions.

The experts from the Karolinska Institutet in Stockholm argued that being asleep during the daytime could impact the brain’s ability to clear waste that builds up during the day. 

But the scientists said it is also possible that brain damage caused by early dementia could be fueling a need for excess sleep. 

It comes as a landmark study last year suggested almost half of all cases of Alzheimer’s disease—the most common cause of dementia—could be prevented by tackling 14 lifestyle factors. 

These included hearing loss, high cholesterol, vision loss and lack of exercise. 

Experts claimed the study, published in the prestigious journal The Lancet, provided more hope than ‘ever before’ that the disorder that blights the lives of millions can be prevented.

Alzheimer’s Disease affects 982,000 people in the UK.

Alzheimer’s Research UK analysis found 74,261 people died from dementia in 2022 compared with 69,178 a year earlier, making it the country’s biggest killer.

One 2019 estimate put the annual death toll at 70,000 people a year with the health issues caused costing the NHS £700million each year to treat.

The WHO puts the annual global death toll from physical inactivity at around 2million per year, making it in the running to be among the top 10 leading causes of global death and disability.