Sleeping in regularly may not be a good idea if you want to keep your brain sharp, according to a new study that found people in their 60s and 70s who slept on average 9 hours or more in a 24-hour period showed a more rapid decline in cognitive function over 3 years than counterparts who slept 6-8 hours.
Decline in cognitive function, such as memory and thinking, is a feature of Mild Cognitive Impairment (MCI), a known risk factor for dementia.
The study, which was conducted by researchers from the University Hospital Madrid and Columbia University in New York, was published online recently in the Journal of Psychiatric Research.
The researchers, led by Dr. Julián Benito-León, looked at data from a large cohort of over 2,700 people in their 60s and 70s who were followed for 3 years.
At the beginning and end of the study period, the participants underwent assessment of brain function using the mini-mental state examination (MMSE), one of a battery of tests used to assess dementia.
Over the 3 years they were followed, the participants also filled in reports that included details of their sleeping patterns.
People who slept on average more than 9 hours a night showed double the amount of cognitive function decline, compared with people who slept 6-8 hours a night.
The results showed that 49% of participants were normal sleepers (they slept on average 6-8 hours over a 24-hour period, this was the "reference group"), 40% were long sleepers (9 or more hours) and 11% were short sleepers (5 hours or less).
Over the follow-up, the MMSE scores declined in all three groups, with long sleepers showing nearly double the amount of decline in cognitive function of normal sleepers.
The authors note that the "difference between long sleepers and the reference group was significant," and the result remained "robust," even when they took into account factors that might influence it, such as age, education, and smoking and drinking habits.
The researchers say further studies are needed to confirm these findings.
It is important to note finding a link does not establish that longer sleep actually causes the mental decline. There could be other explanations, and one question that could be explored is does mental decline cause people to sleep longer?