Some people are genetically programmed to be early birds, others night owls. The amount of DNA influencing this natural preference is dramatically larger than the 24 genes identified in the past, new research indicates.
"Using data from 700,000 individuals, we've found 351 genetic factors that influence whether you prefer mornings or evenings," said Michael Weedon, senior author of the new study , published Tuesday in Nature Communications, and an associate professor at the University of Exeter Medical School in the UK. "These factors affect what time people sleep and wake up."
Better mental health for early risers?
An international collaboration, which included universities and research organizations in the UK, the United States, the Netherlands, Germany and Australia, conducted the study that boasted 250,000 US-based participants enrolled with 23andMe, a company that provides private genomic analyses, and 450,000 people registered with UK Biobank, a nonprofit health resource.
Weedon and his colleagues looked to see which genes were shared among those who had described themselves as a "morning person," an "evening person" or neither.
"Being an early riser or night owl is a genuine feeling of preferring to be active in the morning or the evening," Weedon explained. In reality, most people don't have a strong preference either way, yet we all fall along a bell curve distribution of natural inclination, he added.
The 351 genetic factors identified by the research team included not only those that directly influence the human body clock, also referred to as circadian rhythms, but some genes related to the brain and the retinal tissue in our eyes.
Our human body clock cycle is slightly longer than the 24-hour daily cycle, and so this new eye tissue finding may help explain how our brains use light to daily "reset" and align our body clock with the Earth's cycle. "It could be that these genes in the eye help morning people detect light and 'reset' their body clock more effectively," Weedon noted.
To confirm their results, Weedon and his colleagues examined activity monitor data for 100,000 people and found that morning people wake up about 25 minutes earlier than evening people. Though genes influence sleep timing, they did not affect the quality or duration of sleep, he said.
The newly identified genes may also affect more than our sleep cycles.
Early birds may have better mental health than night owls, Weedon said. He and his colleagues essentially cross-referenced the 351 identified genetic factors against genes known to underlie some mental health conditions and found an association between early rising and subjective well-being, as well as decreased risks for both schizophrenia and depression.
"One speculative explanation for this is that morning people are better aligned with a 9-to-5-type society," Weedon said.
However, night owls scored a small win: The large-scale gene analysis did not show a strong connection between staying up late and developing diabetes or obesity, as theorized in previous research.
Your "chronotype" -- or time of day preference -- affects not only your sleep patterns but your hormone levels and core body temperature. However, it's not entirely innate. Lifestyle factors, including diet, daily activities and exposure to artificial light, influence your chronotype.
What does it mean when lifestyle conflicts with your personal chronotype? Weedon and his colleagues are assessing "whether people who are genetically morning people but who are active in the evening have worse health outcomes than those who are aligned. In the long term, the new biology we've helped uncover may lead to treatments for conditions caused by a disruption of body clocks -- [for example], jet lag," he speculated.
Dr. Zachary Freyberg , an assistant professor of psychiatry and cell biology at the University of Pittsburgh, said the new study is special due to its large size. Both the UK and the Netherlands have been compiling data on patients through their national health services for decades, and "that's a very rich source of information," explained Freyberg, who was not involved in the research. "To further boost the size of the study" by combining national health service records with data obtained from "these new DNA sequencing services like 23andMe is really the next frontier," he said.
The finding of a relationship between time of day preferences and predisposition toward disorders like depression and schizophrenia is also "intriguing," he said, though he cautioned that these results need to be confirmed by other scientific studies.
Freyberg and his colleagues have found that the time of day a patient takes certain antipsychotic drugs (for treating schizophrenia) "affects the likelihood of some of the side effects that these medications have, especially metabolic side effects. Is it possible, then, that the people who tend to be morning people will respond to treatments for psychiatric illness in a different way than people who are evening people?"
The new study does not supply the answer to that question, but at a minimum, it allows researchers to "start asking more questions or better questions," Freyberg said.
Suzanne Hood , an assistant professor of psychology at Bishop's University in Quebec, said the new study shows that "the same areas of DNA" found in previous research "as well as a large number of other regions of DNA" are associated with circadian rhythms.
She criticized how the researchers defined a person's chronotype: "It would be interesting to follow up these findings with other kinds of methods that can track sleep variables with more precision," said Hood, who was not involved in the new study.
"One of my research areas is in age-related changes in circadian rhythms, like the tendency for us to become more like 'morning people' as we get older," she wrote in an email. "I'm curious to understand why this is the case, and if it actually represents a normal pattern of healthy aging, or if it's something that might be a sign of other kinds of age-associated health problems."
Ultimately, the new study suggests "that our chronotype is complex and influenced by many different genes, not just those involved at the very core of our body clock," Hood said.