How often a person takes daytime naps, if at all, is partly regulated by their genes, according to new research led by investigators at Massachusetts General Hospital (MGH) and published in Nature Communications. In this study, the largest of its kind ever conducted, the MGH team collaborated with colleagues at the University of Murcia in Spain and several other institutions to identify dozens of gene regions that govern the tendency to take naps during the day. They also uncovered preliminary evidence linking napping habits to cardiometabolic health. The work was supported by the National Institute of Diabetes and Digestive and Kidney Diseases, the National Heart, Lung, and Blood Institute, MGH Research Scholar Fund, Spanish Government of Investigation, Development and Innovation, the Autonomous Community of the Region of Murcia through the Seneca Foundation, Academy of Finland, Instrumentarium Science Foundation, Yrjö Jahnsson Foundation, and Medical Research Council.
Previously, co-senior author Richa Saxena, PhD, principal investigator at the Saxena Lab at MGH, and her colleagues used massive databases of genetic and lifestyle information to study other aspects of sleep. Notably, the team has identified genes associated with sleep duration, insomnia, and the tendency to be an early riser or “night owl.” To gain a better understanding of the genetics of napping, Saxena’s team performed a genome-wide association study (GWAS), which involves rapid scanning of complete sets of DNA, or genomes, of a large number of people. The goal of a GWAS is to identify genetic variations that are associated with a specific disease or, in this case, habit. For this study, the MGH researchers and their colleagues used data from the UK Biobank, which includes genetic information from 452,633 people. All participants were asked whether they nap during the day “never/rarely,” “sometimes” or “usually.”
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 about daytime sedentary behavior, which can be an indicator of napping. This objective data indicated that the self-reports about napping were accurate. Researchers note that some countries where daytime naps have long been part of the culture (such as Spain) now discourage the habit. Meanwhile, some companies in the United States now promote napping as a way to boost productivity. Several other features of the study bolster its results. For example, the researchers independently replicated their findings in an analysis of the genomes of 541,333 people collected by 23andMe, the consumer genetic-testing company. Digging deeper into the data, the team identified at least three potential mechanisms that promote napping-
The first was sleep propensity: that is, some people need more shut-eye than others. Disrupted sleep is a secondo contributor: a daytime nap can help make up for poor quality slumber the night before. Finally, early morning awakening: people who rise early may “catch up” on sleep with a nap. This leads to think that daytime napping is biologically driven and not just an environmental or behavioral choice. Some of these subtypes were linked to cardiometabolic health concerns, such as large waist circumference and elevated blood pressure, though more research on those associations is needed. Also, a significant number of the genes near or at regions identified by the GWAS are already known to play a role in sleep. One example is KSR2, a gene that had previously found plays a role in sleep regulation. KSR2 codes for a potential protein kinase that, along with its homologue KSR1, serves as a scaffold for the mitogen protein kinase (MAPK) signaling triggered by growth factor receptors.
Among these KSR2 is utilized by the epidermal growth factor receptor, a signaling protein involved in cell proliferation and in the brain may control nervous cells shape and synaptic activities. However, there could be more than this: EGFR signaling was earlier discovered to control sleep patterns and locomotor activities in animals acting on the hypothalamus. Very recently, a paper confirmed that also in zebrafish EGFR signaling is alone enough to drive the sleep patterns in this daytime fish, due to consolidation of the sleep state and decreased sleep latency. Indeed, EGFR inhibitors like erlotinib and gefitinib did reduce sleep as did another growth factor called transforming growth factor beta (TGF-b), when injected in animal brains. It seems a curious serendipity that EGFR, sleep and circadian rhythms seem bound together in some ways, but it may not: chronic sleep disruption and circadian disturbances are associated with higher risk of breast cancer, where the main driver of the malignant proliferation is the EGFR itself.
Is there any coincidence?
- edited by Dr. Gianfrancesco Cormaci, PhD, specialista in Biochimica Clinica.
Dashti HS, Daghlas I et al. Nature Commun 2021; 12(1).
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