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Sleeping too much in middle age can be just  as bad for you as not having enough, it emerged yesterday.

A study of almost 9,000 people found those aged  50 to 64 who slept for less than six hours a night or more than eight had worse memories and decision-making abilities.

But brain power was only reduced for older adults of 65 to 89 if they slept too long.

Get up! Everyone loves a lie-in, but a study suggested doing it regularly when we're older could be bad for us

The dangers of having too  little sleep are well established, but the latest study, carried out by experts at the University of Warwick, indicates that an excess can create similar problems.

Researcher Dr Michelle Miller said the results also suggest that the amount of sleep we need – and its affect on the body and brain – changes with age.

Co-author Professor Francesco Cappuccio claimed getting just the right amount of sleep among the elderly could even prevent the age-related mental decline that can result in dementia.

He added: 'Sleep is important for good health and mental wellbeing. Optimising sleep at an older age may help to delay the decline in brain function seen with age, or may slow or prevent the rapid decline that leads to dementia.'

Dr Miller said: 'Six to eight hours of sleep per night is particularly important for optimum brain function in younger adults. These results are consistent with our previous research, which showed six to eight hours per night was optimal for physical health, including lowest risk of developing obesity, hypertension, diabetes, heart disease and stroke.'

Among the older adults there was a significant relationship between their quality of sleep and cognitive skills. Previous research has shown that getting enough sleep is essential because the brain cleanses itself while the body is asleep, getting rid of harmful toxins accumulated during the day.

Most people can't help nodding off early in life, but research suggests we need to less as we age

But it is less clear why too much sleep would be bad. One theory is that sleeping for long periods reduces the quality of sleep – making a restless period of slumber more likely. Disturbed sleep can impair memory, shrink the brain and create stress. Anyone whose body clock is regularly disrupted, such as nursing mothers and shift staff, is vulnerable.

Existing research has linked oversleeping to a host of medical problems, including diabetes and heart disease. But researchers say the link might be partly because oversleeping is common among the poor and unemployed and those who suffer from depression – groups who are more likely to have health problems.

The findings, published in the journal Plos One, were based on data from 3,968 men and 4,821 women in the English Longitudinal Study of Ageing. They logged the quality and quantity of their sleep over one month.

It comes a month after scientists from Oxford, Cambridge, Harvard, Manchester and Surrey universities said too many people ignored the importance of sleep, creating health and mental problems.

Professor Russell Foster, a neuroscientist at Oxford, said: 'We are the supremely arrogant species. We feel we can abandon four billion years of evolution and ignore the fact that we have evolved under a light-dark cycle. What we do as a species is override the clock. And long-term acting against the clock can lead to serious health problems.'