Too much or too little sleep can raise mental age by two years, dementia study finds

Getting too much – or too little – sleep increases mental age by two years, researchers have shown. There is 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, Canada, shows sleep can play an important part in mental decline.

The studies add 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 United States, 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 suggested cognitive health declined over the long term in some people with sleep problems.

Dr Thies said: “The good news is tools exist to monitor sleep duration and quality and to intervene to help return sleep patterns to normal. There is the possibility that we may also help people preserve their cognitive health.”

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 effect on cognition among older people.

Dr Elizabeth Devore, of Brigham and Women’s Hospital, Boston, and colleagues followed up more than 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 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 in their early forties and those whose sleep changed by two hours per day or more in later life had worse cognitive function than those with no change.

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 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 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 apnœa and disruptions in circadian rhythm.

Another five-year study of 1,300 older women by California University researchers showed participants with sleep apnœa 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 night-time wakefulness were more likely to score worse on tests of cognition and verbal skills.

A third study of 5,000 over-65s showed excessive daytime sleepiness independently increased the risk of cognitive decline.