Regular Daytime Napping Related to Higher Brain Volume, Not Cognitive Function

A moderate causal relationship between daytime napping and increased total brain volume has been confirmed; however, daytime napping does not correlate with improved cognitive outcomes. These are the findings of a study published in the journal Sleep Health.

Observational research has pointed to the existence of a relationship between daytime napping and cognitive function; however, these studies have not indicated a direct causal link between the 2.

Researchers in the United Kingdom and the United States conducted a Mendelian randomization study to determine the causality of the relationship between daytime napping and cognitive function and brain volume. They obtained data from the UK Biobank on the daytime sleeping habits, genetics, and brain morphology and function of 378,932 individuals (mean age, 57). The final study sample included 35,080 individuals with complete neuroimaging, cognitive assessment, and genotype data.

A recent genome-wide association study (GWAS) detected 123 single nucleotide polymorphisms (SNPs) associated with daytime napping. This study used 92 of the 123 SNPs to perform the Mendelian randomization analysis.

Baseline measures of cognitive function included 5 computerized touch-screen cognitive assessments to assess visual memory and reaction time. Structural brain magnetic resonance imaging (MRI) scans aided the analysis of total brain volume and hippocampal volume.

Based on self-reported daytime napping exposures, 57% of the cohort reported that they never or rarely took a daytime nap, 38% reported that they sometimes took a daytime nap, and 5% stated that they usually had a daytime nap.

The researchers observed that participants in the “usual” daytime napping subgroup were more likely to be older, men, current smokers with a history of diabetes and cardiovascular disease on antihypertensives. These individuals who usually took daytime naps more often demonstrated smaller total brain volumes and slower reaction times, compared with the group that never or rarely napped during the day.

They did not find any association between daytime napping and hippocampal volume (95% CI, -0.13 to 0.06); however, they did observe that a genetic predisposition for daytime napping correlated with a 15.80 cm³ larger total brain volume (95% CI, 0.25-31.34). Daytime napping also did not correlate with improved cognitive function with either reaction time (95% CI, 1.00-1.03) or visual memory (95% CI, 0.95-1.05).

The researchers confirmed validation of their findings by analyzing other panels of genetic SNP variants (47 SNPs, 86 SNPs, and 17 SNPs) associated with daytime napping, arriving at similar conclusions.

“This study improves our knowledge of the impact of habitual daytime napping on brain health, which is essential to understanding cognitive impairment in the aging population,” the researchers wrote.

They added that “These findings further our understanding of the relationship between daytime napping frequency, cognitive function, and structural brain outcomes and elucidate the importance of using different measures to better understand how sleep relates to brain health.”

Study limitations included the 77% overlap between exposure and cognitive outcome samples, lack of racial diversity, lack of assessment of length and timing of daytime naps, and the self-reported exposure to daytime napping.