Whether we prefer to get up at the crack of dawn or work late into the night may be influenced by genes connected to the circadian rhythm, a study suggests.
U.S. researchers compared the genomes of nearly 90,000 individuals with their responses to the question of whether they defined themselves as a morning person or evening person - an approach known as a genome-wide association study.
In a paper published today in Nature Communications, the researchers from consumer genetics company 23andMe and San Jose State University report the discovery of 15 gene regions linked to our preference for or against "morningness."
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Seven of these gene regions are located near well-known genes that govern our body clock, or circadian rhythm, which is largely driven by exposure to light, with at least one associated with the relay of information about light to the brain.
Researcher and statistical geneticist Dr David Hinds said another of the genes linked to our preference for mornings or evenings was close to a gene associated with restless legs syndrome, a condition whereby people feel the need to constantly move their legs, particularly when in bed and trying to sleep.
Other genes identified in the study are linked to known sleep disorders, such as narcolepsy.
Hinds says these extreme sleep problems are easier to study because they are more readily classified, and the genes are easier to follow through the generations.
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"There have been fewer studies looking at the normal range of variation among the population, and that was really enabled by having such a large data set as we have at 23andMe," he said.
The study is part of an ongoing project looking for associations between behavioural traits and genes, enabled by the company's access to a huge amount of genetic data from clients seeking information about their ancestry, health risks and other genetic information.
"We've investigated genetics of more than 1,000 different traits like this and among the sleep behaviors we looked at, stood out as having the most interesting genetics," Hinds said.
While the associations between these 15 gene regions and an individual's preference for early mornings were statistically strong, the effect on a person's chance of being a morning person or not was relatively small, with each gene only changing the probability by a few per cent, Hinds said.
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"If you combined the information from all the variants we found and tried to rank or score people as to how likely they are to be a morning or a night person, then compare the top 5 per cent of the people who are at the high end of that scores versus the bottom 5 per cent, there's about a two-fold difference in probability of someone being a morning person."
The analysis identified other traits associated with sleep preferences such as female gender, increasing age, body mass index, insomnia, sleep walking and depression. But there was no apparent genetic link underlying these associations.
Commenting on the study, molecular chronobiologist Dr. Maria Comas from the Woolcock Institute said the paper added to the genes already known to influence sleep habits through the circadian rhythm.
However, she pointed out that many well-known circadian genes did not show an association with morningness.
"This is interesting, because these are in the core of the circadian clock molecular machinery," Comas said.
This originally appeared on ABC Science Online.