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.

The genes flagged in the study play a wide variety of roles in the body.

Many seem to play a role in brain tissues, and others are already known to be central to the body’s circadian rhythm. A few were active mainly in the retina, and the people who possessed an uncommon version of one of these genes had an increased chance of being night owls, said Samuel Jones, a researcher at the University of Exeter and the study’s lead author. That could imply a potential a connection between how the eye responds to sunlight and when a person sleeps.

Another gene was involved in the body’s processing of caffeine and nicotine, two of our species’ favorite stimulants. Continued study of these and the other genes could provide leads for future work on the biology of sleep timing.

“The most interesting ones are the ones where we don’t know what it is,” said Dr. Weedon.

When the researchers crunched the numbers on chronotype’s connection to mental health, they also found that self-identified morning people reported a higher level of general well-being. People in this group also were less likely to report having depression or schizophrenia, in line with epidemiological studies suggesting that evening people struggle with mental health.

The researchers wonder whether having a lifestyle that aligns with one’s chronotype may be more important in mental and physical health than whether you are merely a morning or night person. In future work, they are hoping to see whether morning people who are required to stay up late for their jobs or other commitments — perhaps similar to night owls who must rise early for 9-to-5- jobs — show higher levels of mental disorders than their well-aligned counterparts.

“Perhaps evening people are constantly fighting their natural clock, which might have unintended consequences farther down the line,” said Dr. Jones.

Earlier reporting on sleep
Why It Hurts to Lose Sleep

The Purpose of Sleep? To Forget, Scientists Say

Hot Heads: Why Mammals Need R.E.M. Sleep