Daytime napping may help to preserve brain health by slowing the rate at which our brains shrink as we age, suggests a new study published in Sleep Health.
The study, led by researchers at University College London and the University of the Republic in Uruguay, analyzed data from people aged 40 to 69 and found a causal link between habitual napping and larger total brain volume—a marker of good brain health linked to a lower risk of dementia and other diseases.
“Our findings suggest that, for some people, short daytime naps may be a part of the puzzle that could help preserve the health of the brain as we get older,” says senior author Victoria Garfield, PhD, MRC Unit for Lifelong Health and Ageing at University College London, in a release.
Previous research has shown that napping has cognitive benefits, with people who have had a short nap performing better in cognitive tests in the hours afterward than counterparts who did not nap.
The new study aimed to establish if there was a causal relationship between daytime napping and brain health.
Using Mendelian randomization, they looked at 97 snippets of DNA thought to determine people’s likelihood of habitual napping. They compared measures of brain health and cognition of people who are more genetically “programmed” to nap with counterparts who did not have these genetic variants, using data from 378,932 people from the UK Biobank study, and found that, overall, people predetermined to nap had a larger total brain volume.
The research team estimated that the average difference in brain volume between people programmed to be habitual nappers and those who were not was equivalent to 2.6 to 6.5 years of aging.
But the researchers did not find a difference in how well those programmed to be habitual nappers performed on three other measures of brain health and cognitive function: hippocampal volume, reaction time, and visual processing.
“This is the first study to attempt to untangle the causal relationship between habitual daytime napping and cognitive and structural brain outcomes,” says lead author and PhD candidate Valentina Paz from the University of the Republic and MRC Unit for Lifelong Health and Ageing at University College London, in a release. “By looking at genes set at birth, Mendelian randomization avoids confounding factors occurring throughout life that may influence associations between napping and health outcomes. Our study points to a causal link between habitual napping and larger total brain volume.”
The genetic variants influencing the likelihood to nap were identified in an earlier study looking at data from 452,633 UK Biobank participants. The study, led by Hassan Dashti, PhD, of Harvard University and Massachusetts General Hospital, also an author on the new study, identified the variants on the basis of self-reported napping, and this was supported by objective measurements of physical activity recorded by a wrist-worn accelerometer.
In the new study, researchers analyzed health and cognition outcomes for people with these genetic variants as well as several different subsets of these variants, adjusted to avoid potential bias, for instance avoiding variants linked to excessive daytime sleepiness.
Genetic data and magnetic resonance imaging scans of the brain were available for 35,080 individuals drawn from the larger UK Biobank sample. In terms of study limitations, the authors noted that all of the participants were of white European ancestry, so the findings might not be immediately generalizable to other ethnicities.
While the researchers did not have information on nap duration, earlier studies suggest that naps of 30 minutes or less provide the best short-term cognitive benefits, and napping earlier in the day is less likely to disrupt night-time sleep.
Previous research looking at the UK and the Netherlands found that nearly a third of adults aged 65 or over had a regular nap.