Genetic variations that are associated with a person’s preference for the morning as opposed to the evening are identified in a new paper published in Nature Communications. The study defines people with a preference for the morning as those who choose to rise and rest early, whereas people with a preference for the evening choose a cycle later in the day.

Circadian rhythms, which follow a 24-hour cycle, affect many biological processes including a person’s natural preference for the morning or the evening. Several genes whose products regulate circadian rhythm are already known, but it was previously unclear whether these were linked to personal preference for the morning.

David Hinds and colleagues perform a genome-wide association study (GWAS) of 89,283 individuals. The individuals reported a preference for the morning or the evening in a web survey. The authors identify specific genetic variations associated with healthy individuals’ preference for the morning, and show that some of the identified genetic variations are located near genes that are known to regulate circadian rhythm. They also find genetic variations near genes that control light-sensing and show that they are associated with preference for the morning.

Although the study also shows that sleep disorders and higher body mass index often accompany a self-reported preference for the morning, no evidence for a causal relationship was found. The authors propose that identification of these genetic variations will aid future research into circadian rhythms and into sleep and related disorders.