Early to bed and early to rise is a maxim that’s easy to follow for some people, and devilishly hard for others.

Now, in a study published Tuesday in Nature Communications, researchers curious about the genetic underpinnings of chronotype — whether you are a morning person, a night owl or somewhere in between — looked at about 700,000 people’s genomes. They identified 351 variations that may be connected to when people go to bed. While these variants are just the beginning of exploring the differences in chronotypes, the study goes on to suggest tantalizing links between chronotype and mental health.

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The researchers drew on data from 23andMe, the genetic testing company, and the UK Biobank, which tracks hundreds of thousands of volunteer subjects in Britain, about 85,000 of whom wear activity monitors that record their movements.

Those data were key, said Michael Weedon, a bioinformaticist at University of Exeter in England and an author of the new paper; earlier studies had relied only on people’s subjective opinions of whether they were morning people. Using the activity monitors, however, the team was able to confirm that self-reported morning people did go to sleep earlier — and people with the most morning-linked gene variants went to bed 25 minutes earlier than people with the fewest. Morning people did not sleep longer or better than night people; all that differed was the time that they went to sleep.