A new study led by researchers at University College London and the Universidade de la Repubblica in Uruguay suggests that daytime naps can help keep the brain healthy by slowing the rate at which brain size shrinks with age. The study, published in the journal SleepHealth, evaluated data from people aged 40 to 69 years.
The findings show that regular naps are directly linked to increased brain volume, which is an indicator of brain health and is associated with reduced rates of dementia and other diseases.
"Our findings suggest that for some people, short daytime naps may be part of the puzzle that helps keep the brain healthy as we age," said the study's senior author, Dr. Victoria Garfield (Unit of Lifespan Health and Aging, UCLA Medical Research Center).
Previous research has shown that naps have cognitive benefits, with people who take short naps performing better on cognitive tests several hours later than those who don't nap.
New research aims to determine whether there is a causal link between daytime naps and brain health. Using a technique called Mendelian randomization, they looked at 97 DNA fragments that they thought could determine a person's likelihood of napping. Using data from 378,932 people in the UK Biobank study, they compared the brain health and cognitive abilities of those who were genetically "programmed" to prefer napping to those who did not have these genetic variants.
The team estimated that the average difference in brain size between those who habitually nap and those who do not is equivalent to 2.6 to 6.5 years of aging.
However, the researchers did not find any difference in the performance of those who were programmed to be habitual nappers on three other measures of brain health and cognitive function - hippocampal volume, reaction time and visual processing.
"This is the first study to try to unravel the causal link between habitual daytime napping and cognitive and brain structural outcomes," said lead author Valentina Paz, a PhD student at the University of the Republic of Uruguay and the MRC Center for Lifelong Health and Aging at University College London. Genetic, Mendelian randomization at birth avoids lifelong confounding factors that may influence the relationship between napping and health outcomes. "Our study shows a causal link between habitual naps and increased overall brain size. I hope studies like this showing the health benefits of short naps will help reduce the stigma surrounding daytime naps."
An earlier study found genetic variants that influence our likelihood of napping in data from 452,633 UK Biobank participants. The research was led by Dr. Hasan-Dashti (Harvard University and Massachusetts General Hospital), one of the authors of the new study. This study identified variants based on self-reported napping, supported by objective measures of physical activity recorded with a wrist-worn accelerometer.
In the new study, the researchers analyzed health and cognitive outcomes in people with these gene variants and in several different subsets of those variants, adjusting to avoid potential biases, such as avoiding variants associated with excessive daytime sleepiness.
Genetic data and brain magnetic resonance imaging (MRI) scans of 35,080 people drawn from the larger UK Biobank sample are available. Regarding the limitations of the study, the authors noted that all participants were of white European descent, so the findings may not be immediately generalizable to other ethnicities.
Although the researchers had no information on nap duration, earlier studies have shown that naps of 30 minutes or less provide the best short-term cognitive benefits, and that naps taken earlier in the day are less likely to disrupt nighttime sleep.
Previous research in the UK and the Netherlands found that nearly a third of adults aged 65 or older took regular naps.