Perhaps you are one of those peppy people who spring into action with the sunrise.
Or perhaps you find that sort of person exhausting, and you do your best work in the evening.
As with so many other human traits, genes seem to play a key role in the answer.
In a new study of 450,000 people, researchers identified 351 genetic variants that were associated with chronotype -- the scientific term for when a person prefers to sleep and wake.
Individually, each variant was found to have just a slight connection with whether participants preferred the morning or evening. But collectively, the various genetic markers made an appreciable difference, the authors reported recently in Nature Communications. The 351 telltale snippets of DNA consisted of 24 that had been previously linked to chronotype and 327 new ones.
Some of the new findings were plausible, in that the genes in question were known to be connected to how the body regulates various hormones, while others had no apparent connection to our daily rhythms. Now comes the hard work of sorting out cause and effect, and someday determining which of the genes might be useful in diagnosis and treatment.
The study was led by researchers at the University of Exeter in England, with collaborators from the University of Pennsylvania, among other institutions.
"We know from these genetic studies that some of those are going to be real, informative associations, and some of them are just random," said coauthor Philip R. Gehrman, an associate professor in the psychiatry department at Penn's Perelman School of Medicine. "It's a matter of working through them and trying to figure out which ones are the meaningful ones."
Contrary to what has been reported in some previous studies, the researchers found that night owls were no more likely than morning people to suffer from obesity or Type-2 diabetes.
The authors speculated that perhaps the true culprit in those diseases is not whether someone is a night owl, but whether there is a mismatch between a person's work schedule and natural sleep rhythm -- say, a morning person who is regularly assigned to work the graveyard shift.