Napping during the day is partly controlled by a person's genetics – and is not simply a behavioural choice.
Previous research has shown that when people are sleep deprived, daytime naps can have short term benefits on performance and alertness. However, it is not fully understood whether habitual napping has any longer term impacts on chronic disease risk.
Dr Hassan Saeed Dashti, who co-led the study, said, 'Napping is somewhat controversial. It was important to try to disentangle the biological pathways that contribute to why we nap.'
To better understand the causes and consequences of daytime naps, researchers at the Centre for Genomic Medicine at Massachusetts General Hospital (MGH) performed a genome-wide association study (GWAS) to analyse the entire genomes of nearly half a million people using the UK Biobank dataset of European ancestry.
They aimed to identify genetic variations that explain why different people have different tendencies to nap during the day.
All participants were asked whether they nap during the day 'never/rarely,' 'sometimes' or 'usually.' A subset of participants wore activity monitors, which provided more objective data about daytime sedentary behaviour, which can be an indicator of napping.
Published in the journal Nature Communications, the researchers discovered a total of 123 regions in the human genome that are associated with increased daytime napping, with many of the gene regions already known to play a role in sleep and sleep disorders.
Genetic variations were found in the neuropeptide orexin pathway. 'This pathway is known to be involved in rare sleep disorders like narcolepsy, but our findings show that smaller perturbations in the pathway can explain why some people nap more than others' said co-author Iyas Daghlas from Harvard Medical School, Massachusetts.
Further analysis revealed several distinct clusters of genes that are associated with napping, which allowed the researchers to suggest that at least three separate mechanisms are involved in promoting daytime napping.
One mechanism is related to 'disrupted sleep', whereby a daytime nap could make up for a poor quality night-time sleep. A second was related to 'early morning awakening', as some people who rise early may need to catch up on sleep with a nap. The third mechanism was related to 'sleep propensity', meaning that some people simply need more sleep than others.
Dr Dashti reasoned, 'This tells us that daytime napping is biologically driven and not just an environmental or behavioural choice.'
Notably, some of the gene regions found to be linked to nap requirements were also linked to cardiometabolic health concerns, such as increased waist circumference and raised blood pressure. This led the researchers to suggest that daytime napping could be implicated in medical issues such as hypertension and obesity, although additional work is needed to confirm a direct link.
Nonetheless, the researchers also suggested that better understanding the links between daytime naps and chronic conditions may reveal pharmacological targets for such conditions.