Daytime napping may help to preserve brain health by slowing the rate at which a person's brain shrinks as they age, a new study has suggested.
Researchers from University College London (UCL) and the University of the Republic in Uruguay explained that daytime napping – brief daytime bouts of sleep – was a "universal and prevalent behaviour". Most under 3-year-olds nap, with napping becoming less common during school age and adulthood, and then increasing again in older adults.
"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 afterwards than counterparts who did not nap," the authors of the study pointed out.
However, it remained "elusive" whether these associations were causal, so the researchers set out to establish if there was a causal relationship between daytime napping and brain health.
Habitual Daytime Napping and the Brain
For the study, published in Sleep Health, researchers analysed the data of 378,932 people – mean age 57 years - from the UK Biobank. Of the sample, 57% 'never/rarely' had a daytime nap, whilst 38% and 5% reported 'sometimes' and 'usually' having a daytime nap, respectively. Those who 'usually' had a daytime nap were older, less likely to be female, more likely to be deprived, to be a current smoker, on antihypertensives, have a diagnosis of diabetes, and have prevalent cardiovascular disease, explained the authors.
The researchers used Mendelian randomisation to investigate the relationship between habitual daytime napping and cognitive/structural brain outcomes. Measures of brain health and cognition of people who were more genetically "programmed" to nap were compared with counterparts who did not have these genetic variants. The outcomes were total brain volume, hippocampal volume, reaction time, and visual memory.
To assess visual memory, participants were asked to correctly identify matches from six pairs of cards after they had memorised their positions. Reaction time was recorded as the mean time taken by participants to correctly identify matches in a 12-round game of the card game 'Snap'.
Valentina Paz, a PhD candidate from the University of the Republic (Uruguay) and the MRC Unit for Lifelong Health & Ageing at UCL and lead author, said: "This is the first study to attempt to untangle the causal relationship between habitual daytime napping and cognitive and structural brain outcomes."
Short Daytime Naps Could Preserve Brain Health
The researchers found a "modest" causal association between habitual daytime napping and larger total brain volume.
"Overall, people predetermined to nap had a larger total brain volume," they said, and postulated this "could suggest that napping regularly provides some protection against neurodegeneration through compensating for poor sleep".
They pointed out that genetic liability to daytime napping was associated with 15.8 cm3 larger total brain volume, which they estimated was equivalent to 2.6 to 6.5 years of ageing.
Dr Victoria Garfield, from the MRC Unit for Lifelong Health and Ageing at UCL, and senior author 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."gg
However, the researchers did not find an association between habitual daytime napping and hippocampal volume, reaction time, or visual memory, which the authors said "surprised" them. They suggested that "more reliable" cognitive measures may be required to identify these effects.
Reduce Daytime Napping Stigma
The authors acknowledged some limitations of their study, which included that all of the participants were of white European ancestry, so the findings might not be immediately generalisable to other ethnicities. Also, they emphasised that whilst they did not have information on nap duration, earlier studies suggested that naps of 30 minutes or less provided the best short-term cognitive benefits, and napping earlier in the day was less likely to disrupt night-time sleep.
Commenting on the study, Professor Tara Spires-Jones, president of the British Neuroscience Association, group leader at the UK Dementia Research Institute, and deputy director of the Centre for Discovery Brain Sciences at the University of Edinburgh, cautioned that the study had limitations. "The napping habits of UK Biobank participants were self-reported, which might not be entirely accurate," she alerted.
However, she stressed that even with limitations, the study was "interesting" because it added to the data that indicated sleep was "important for brain health".
Dr Garfield hoped that "studies such as this one showing the health benefits of short naps can help to reduce any stigma that still exists around daytime napping".
The study was funded by Programa de Desarrollo de las Ciencias Básicas, Agencia Nacional de Investigación e Innovación, Comisión Sectorial de Investigación Científica, Comisión Académica de Posgrados, National Heart, Lung, and Blood Institute, Diabetes UK, British Heart Foundation. The authors declared no conflicts of interests.