Scientists call a lark the type of person who gets out of bed easily and finds the highest productivity of the day in the morning. Their chronotype – this is the technical name given to each pattern of sleep and activity – is the opposite of that of owls, who work better at night and go to bed and get up late, according to social conventions. A new study, published today in Nature Communications, reveals that those who are genetically programmed to wake up soon are less likely to suffer from mental illnesses such as depression or schizophrenia. They also say they feel better than others.
It was known that the genetics of each individual conditions their ability to follow one or the other schedule. But previous research had only identified a handful of genes relevant to chronotype, and those studies that have sought a link with health have only found correlations, not causality. The new work, which has analyzed the genome of 697,828 people, concludes that there are at least 351 genes that predispose a person to be an owl or a lark, hundreds more than the 24 that were known. By determining which genetic variants alondras share, researchers have been able to establish a cause-and-effect relationship between being early and enjoying greater mental health.
The international research team, led by biologists from the University of Exeter (United Kingdom), analysed the genomes of British citizens collected in the UK Biobank research database and also data from the US private company 23andMe, which sells personal genetic tests. Each participant had previously indicated their sleep pattern in a survey. However, the scientists were looking for a more objective measure of chronotype, so they included in the analysis data from 85,760 people whose sleep hours were recorded with an activity bracelet.
The study confirms that those genetically predisposed to be larks fell asleep an average of 25 minutes earlier in the day than owls. They obtained this measure by comparing the 5% of people who had more early genes in their DNA with the 5% who had less. The time difference between one end of the spectrum and the other seems modest, but is statistically significant. There were no differences, however, in sleep duration or quality.
Although morning dwellers reported better overall well-being and were found to have fewer cases of depression and schizophrenia, researchers have not found that being an early riser protects against other diseases, such as diabetes or obesity. “This was a little surprising,” says University of Exeter biologist Samuel Jones, one of the study authors. “A lot of research has found that owls have poorer metabolic regulation and perhaps a risk of diabetes and obesity. But these studies tend to be correlated; we have been able to infer cause and effect by using genetics.
Social jet lag
One possibility for explaining previous results is that there is a third factor common to those who go to bed late and to those who suffer from these diseases, Jones explains. But it’s also possible that being an owl isn’t intrinsically bad for your health, but what’s harmful is getting up early for social and work commitments when genetics predisposes you to the opposite. This produces imbalances in the internal biological clocks, or social jet lag, and has proven negative consequences for health.
Precisely, many of the genes identified in this study are responsible for regulating the body’s circadian clocks, the biochemical processes that govern the periodicity of cellular activities. Jones points out that “the circadian rhythm can be trained to a certain extent,” especially by trying to maintain a routine for going to bed and waking up at the same time. “If you’re an afternoon person, you can advance much of the way to being a morning person,” explains the researcher. But he adds: “The genes will stop you in the final stretch because the owls have an internal clock that runs a little slower. That’s genetic and you can’t change it.
Maria José Martínez, the coordinator of the Chronobiology Group of the Spanish Sleep Society (SES), who did not participate in this study, states that “if we lived in a society in which everyone could organize their schedules (work, sleep, etc.) freely, it would be indifferent for health to be morning or afternoon”. But Martínez, who is also manager of the circadian consultancy company Kronohealth, adds that for owls this would only be true “as long as we use intense artificial light during waking hours and avoid light during sleeping hours”.
The incidence of light is an important factor because it stops the production of melatonin, a hormone that promotes sleep. A tip to advance the routine when the body does not ask you is to expose yourself to light, natural or lamp, early in the morning, even before waking up. Interestingly, Jones and his colleagues have found that some of the 351 genes associated with the chronotype are expressed in the cells of the retina in the eye, something that suggests that “larks probably perceive light slightly differently than owls,” according to the study author.
Other genes they have identified are expressed in the hypothalamus, a region of the brain with sleep and wakefulness regulation functions, some are involved in insulin metabolism and some influence the processing of stimulant substances, such as caffeine and nicotine. All this points to intrinsic physiological differences between owls and larks, but detailed studies will be necessary to unravel how or why each of these genetic variants affects the chronotype and by extension mental health.