Do you find yourself getting much less sleep as you reach middle age? Or are you sleeping more? Either way it could mean your brain is prematurely ageing, equivalent to a decline of up to seven years.
In a study of more than 5,400 people aged 45-69, scientists found that there was significant decline in brain power in those people who had, over the course of a five-year period, changed the amount they slept from the optimum six to eight hours per night.
"The main result to come out of our study was that adverse changes in sleep duration appear to be associated with poorer cognitive function in later middle age," said Jane Ferrie of the department of epidemiology and public health at University College London Medical School, who led the study. The results are published in the 1 May edition of the journal Sleep.
The participants were asked about their sleeping habits in 1997-1999 to establish baseline numbers and again in 2002-2004 for a follow-up. Their cognitive functions were monitored using a range of tests that included measurements of memory, vocabulary and reasoning. They also carried out the mini mental state examination, designed to test for the early signs of dementia.
Women who slept for about seven hours a night and men who slept for six to eight hours scored the highest. Sleeping much less than six hours or much more than eight was associated with lower scores.
The researchers then looked at how people's sleep patterns had changed. At the time of the follow-up in 2002-2004, 7.4% of the women and 8.6% of the men said they had increased the amount they had been sleeping compared with their baseline amounts in 1997-1999.
When compared with people whose sleep patterns had not changed since their baseline, those who had been sleeping more showed lower scores on five of the six cognitive function tests. Only memory was left unchanged.
In 25% of the women and 18% of men, the amount of sleep had dropped significantly from their baseline figures. These people had lower scores in reasoning, vocabulary and global cognitive tests.
The results were part of the Whitehall Study II, which has followed more than 10,000 British civil servants since 1985 to investigate how health is affected by social conditions.
Good-quality sleep is important for proper functioning and wellbeing, said the researchers. Bad sleep patterns have been linked to detriments in a wide range of mental and physical health factors. Too much sleep has also been linked to early death.
"The detrimental effects of too much, too little and poor quality sleep on various aspects of health have begun to receive more attention," said Ferrie.
"Given that our 24/7 society increasingly impinges on the lives of many people, it is important to consider what effects changes in sleep duration may have on health and wellbeing in the long term."
