Daytime napping may help to preserve brain health by slowing the rate at which that organ shrinks as we age, according to scientists, whose study analysed 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.
The study was led by researchers at University College London and the University of the Republic in Uruguay, and published in the journal Sleep Health.
Senior author Dr Victoria Garfield (MRC Unit for Lifelong Health & Ageing at UCL) said: “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.”
Previous research has shown that napping has cognitive benefits, with people performing better in cognitive tests in the hours after a short nap than counterparts who did not nap.
The latest study aimed to establish if there were a causal relationship between daytime napping and brain health.
Using a technique called Mendelian randomisation, 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. They 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 ageing.
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.
Lead author and PhD candidate Valentina Paz (University of the Republic (Uruguay) and MRC Unit for Lifelong Health & Ageing at UCL) said: “This is the first study that attempts to untangle the causal relationship between habitual daytime napping and cognitive and structural brain outcomes. By looking at genes set at birth, Mendelian randomisation 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 our likelihood to nap were identified in an earlier study looking at data from 452 633 UK Biobank participants.
That study, led by Dr Hassan Dashti (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 analysed 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 (MRI) brain scans were available for 35 080 individuals drawn from the larger UK Biobank sample.
Study limitations included that all of the participants were of white European ancestry, so the findings might not be immediately generalisable 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 older had a regular nap.
Study details
Is there an association between daytime napping, cognitive function, and brain volume? A Mendelian randomisation study in the UK Biobank.
Valentina Paz, Hassan Dashti, Victoria Garfield.
Published in Sleep Health on 20 June 2023
Abstract
Objectives
Daytime napping has been associated with cognitive function and brain health in observational studies. However, it remains elusive whether these associations are causal. Using Mendelian randomisation, we studied the relationship between habitual daytime napping and cognition and brain structure.
Methods
Data were from UK Biobank (maximum n = 378,932 and mean age = 57 years). Our exposure (daytime napping) was instrumented using 92 previously identified genome-wide, independent genetic variants (single-nucleotide polymorphisms, SNPs). Our outcomes were total brain volume, hippocampal volume, reaction time, and visual memory. Inverse-variance weighted was implemented, with sensitivity analyses (Mendelian randomisation-Egger and Weighted Median Estimator) for horizontal pleiotropy. We tested different daytime napping instruments to ensure the robustness of our results.
Results
Using Mendelian randomisation, we found an association between habitual daytime napping and larger total brain volume (unstandardised ß = 15.80 cm3 and 95% CI = 0.25; 31.34) but not hippocampal volume (ß = −0.03 cm3 and 95% CI = −0.13;0.06), reaction time (expß = 1.01 and 95% CI = 1.00;1.03), or visual memory (expß = 0.99 and 95% CI = 0.94;1.05). Additional analyses with 47 SNPs (adjusted for excessive daytime sleepiness), 86 SNPs (excluding sleep apnea), and 17 SNPs (no sample overlap with UK Biobank) were largely consistent with our main findings. No evidence of horizontal pleiotropy was found.
Conclusions
Our findings suggest a modest causal association between habitual daytime napping and larger total brain volume. Future studies could focus on the associations between napping and other cognitive or brain outcomes and replication of these findings using other datasets and methods.
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