Scientists say that having regular lie-ins in old age can bring on dementia.

Getting too much or too little sleep increases your mental age by two years, researchers showed.

There is what scientists call the “goldilocks zone”, seven hours, which is neither too much nor too little.

A series of studies presented at an Alzheimer’s conference in Vancouver shows when it comes to mental decline sleep can play an important part.

It adds to evidence that poor sleep quality and quantity in the elderly increases the risk of a range of illnesses – including heart disease and diabetes.

Dr. William Thies, of the Alzheimer’s Association in the US who organized the annual meeting, said: “We know sleep patterns change as people age and that poor sleep affects overall health.

“What we don’t know for certain is whether poor sleep has long-term consequences on cognitive function.”

He said the latest research suggests cognitive health declines over the long term in some people with sleep problems.

Getting too much or too little sleep increases your mental age by two years

Dr. William Thies said: “The good news is tools already exist to monitor sleep duration and quality and to intervene to help return sleep patterns to normal.

“If we do this, there is the possibility that we may also help people preserve their cognitive health, but that needs to be tested.”

Previous studies have suggested sleep duration shorter or longer than the recommended seven hours per day may increase the risk of cardiovascular disease and type 2 diabetes.

But little research has been carried out on its affect on cognition among older individuals.

So Dr. Elizabeth Devore, of Brigham and Women’s Hospital, Boston, and colleagues followed up over 15,000 retired nurses aged 70 or older every other year for six years.

Those who slept five hours or less or nine hours or more per day had lower average cognition than those who slept seven hours.

Too little or too much sleep was cognitively equivalent to ageing by two years.

The women were recruited for the long-term study in their early 40s and those whose sleep changed by two hours per day or more in later life had worse cognitive function than those with no change, independent of their initial duration.

In a small sample of women who gave blood samples declining ratios of proteins that suggest Alzheimer’s disease brain changes were present in those who slept less or more than seven hours.

Dr. Elizabeth Devore said: “Our findings support the notion that extreme sleep durations and changes in sleep duration over time may contribute to cognitive decline and early Alzheimer’s changes in older adults.

“The public health implications of these findings could be substantial, as they might lead to the eventual identification of sleep- and circadian- based strategies for reducing risk of cognitive impairment and Alzheimer’s.”

As people age, they are more likely to develop problems with sleeping, such as insomnia, sleep apnea and disruptions in circadian rhythm that follow a 24-hour cycle.

Another five year study of 1,300 older women by California University researchers showed participants with sleep apnoea had more than twice the odds of developing dementia compared with those who did not have sleep-disordered breathing.

Women who developed a disruption of their body clock were also at increased risk of dementia and those with greater nighttime wakefulness were more likely to score worse on tests of global cognition and verbal skills.

Dr. Kristine Yaffe said: “We believe these results indicate the relationship between sleep disordered breathing and dementia may be connected to the decrease in oxygen associated with sleep apnoea and not to disrupted patterns of sleep.

“Overall, our findings support a relationship between sleep disturbances and cognitive decline in late age.

“They suggest health practitioners should consider assessing older people with sleep disorders for changes in cognition.”

She said with additional long-term research treatment of sleep disorders may be a promising method of delaying the development of dementia.

A third study of nearly 5,000 over 65 year-olds showed excessive daytime sleepiness – reported by 17.9% of participants – independently increased the risk of cognitive decline along with difficulty maintaining sleep – reported by 63.5%.

Dr. Claudine Berr, of the National Institute of Health and Medical Research in Montpellier, France, said: “These results suggest excessive daytime sleepiness may be an early predictor of cognitive decline and sleep complaints should be adequately evaluated in older persons.”