How often a person takes naps during the day, if at all, is regulated in part by their genes, according to new research led by researchers at Massachusetts General Hospital (MGH). In this study, the largest of its kind ever conducted, the MGH team collaborated with colleagues from the University of Murcia in Spain and several other institutions to identify dozens of gene regions that tend to take naps during the day. They also uncovered preliminary evidence linking sleep habits to cardiometabolic health.
“Napping is somewhat controversial,” said Hassan Saeed Dashti, co-lead author of the report with Iyas Daghlas, a medical student at Harvard Medical School (HMS). Dashti notes that some countries where long daytime naps were part of the culture (such as Spain) are now discouraging the practice. Meanwhile, some companies in the United States are now promoting naps as a way to increase productivity. “It was important to try to untangle the biological pathways that contribute to why we nap,” says Dashti.
Previously, co-senior author Richa Saxena, PhD, principal investigator in the Saxena Lab at MGH, and her colleagues used huge databases of genetic and lifestyle information to study other aspects of sleep. Notably, the team identified genes linked to sleep duration, insomnia and the tendency to get up early or be "night owls." To better understand the genetics of napping, Saxena's team and co-senior author Marta Garaulet, PhD, of the Department of Physiology at the University of Murcia, conducted a genome-wide association study (GWAS), quickly performing complete sets of DNA, or genomes, of a large number of people. The purpose of a GWAS is to identify genetic variations associated with a specific disease or, in this case, habit.
For this study, the MGH researchers and their colleagues used data from the British Biobank, including genetic information from 452,633 people. All participants were asked whether they 'never/rarely', 'sometimes' or 'usually' take a nap during the day. The GWAS identified 123 regions in the human genome that are associated with daytime napping. A subset of participants wore activity monitors called accelerometers, which provide data on daytime sedentary behavior, which may be an indicator of napping. These objective data indicated that the napping self-reports were accurate. “That gave an extra layer of confidence to what we found,” says Dashti.
Several other features of the study bolster the results. For example, the researchers independently replicated their findings in an analysis of the genomes of 541,333 people. It is also already known that a significant number of the genes near or in regions identified by the GWAS play a role in sleep. One example is KSR2, a gene that the MGH team and collaborators previously found plays a role in sleep regulation.
By digging deeper into the data, the team identified at least three possible mechanisms that promote napping:
Sleep tendency: some people need more closed eyes than others.
Disturbed sleep: a nap during the day can help make up for poor sleep the night before.
Wake up early: people who get up early can catch up on their sleep with a nap.
“This tells us that daytime napping is biologically driven and not just an environmental or behavioral choice,” Dashti says. Some of these subtypes were associated with cardiometabolic health problems, such as a large waist circumference and elevated blood pressure, although more research on those associations is needed.
In addition, several gene variants linked to napping were already associated with signaling by a neuropeptide called orexin, which plays a role in wakefulness. “This pathway is known to be involved in rare sleep disorders such as narcolepsy, but our findings show that smaller perturbations in the pathway may explain why some people nap more than others,” says Daghlas.
