Living organisms are governed by an internal biological clock known as the circadian rhythm, and as the Earth rotates every 24 hours, this clock aligns with the cycle of day and night. In a new study, scientists reveal this process has a strong link to mental health. There’s a larger chunk of the genome dedicated to body clocks than previously realized, and those genes are linked to others that determine a person’s overall state of well-being.

In the paper, released Tuesday in Nature Communications, medical researchers explain that being a “morning person” or “evening person” is a behavioral consequence of a person’s underlying circadian rhythm, which is shaped by their genes. Some genes, they found, can even shift a person’s natural waking time by up to 25 minutes. In the study, they investigated whether these “chronotype” genes were associated with any genes related to mental health. They found that people who tend to wake up earlier are more likely to experience a greater sense of well-being and a lower risk of schizophrenia and depression.

“Our work indicates that part of the reason why people are up with the lark while others are night owls is because of differences in both the way our brains react to external light signals and the normal functioning of our internal clocks,” lead author and University of Exeter research fellow Samuel Jones, Ph.D., explained Tuesday. “These small differences may have potentially significant effects on the ability of our clock to keep time effectively, potentially altering risk of both disease and mental health disorders.”

The circadian rhythm orchestrates many biological processes, including digestion, immune function, and blood pressure, all of which rise and fall at specific times of the day. YassineMrabet, CC BY-SA

The team looked through genomic data from 250,000 American 23andMe users and 450,000 individuals involved in a UK Biobank study to find a link between the genes that govern the body clock and those that influence mental health. All individuals reported whether they were a “morning person” or an “evening person,” and because 85,000 of the Biobank participants wore activity trackers on their wrists, the team could tell who was getting up early.

It was previously thought that there were 24 genes that influenced the time an individual wakes up. The new findings, however, showed that there are actually 351. These genes, the team explains, are not only central to our body clocks but are also expressed in the brain and in the eye’s retinal tissue, which could explain how the brain detects light to “reset” the body clock each day.

The discovery and cloning of the CLOCK gene in the 1990s elevated circadian rhythm research beyond fruit flies. UTSW

Analyzing for patterns of chronotype genes and mental health genes, the team found a small but statistically significant correlation between the genetic variants that increased the chance of being a night owl and those that increased the risk of mental health disorders, including depression and schizophrenia. (In addition, while other studies have proposed a genetic link between being a night owl and the development of obesity and type 2 diabetes, the findings of this study didn’t support that link.)

Importantly, the findings do not mean that people who don’t get enough sleep are more likely to be depressed, although that link has been demonstrated in other studies. This study had to do with the genes that influence sleep timing, not quality or duration, and it appears that “morningness” genes tend to occur together with genes for good mental health. As the team writes, “we show that being a morning person is causally associated with better mental health but does not affect BMI or risk of type 2 diabetes.”

While they establish a causal link between the mental health genes and morningness genes using a statistical technique called Mendelian Randomization, the team acknowledges that more research is needed to understand the extent to which the body clock genes can influence our well-being.

“The large number of people in our study means we have provided the strongest evidence to date that ‘night owls’ are at higher risk of mental health problems, such as schizophrenia and lower mental well-being, although further studies are needed to fully understand this link,” said University of Exeter Medical School professor Mike Weedon, Ph.D.

Scientists see the understanding of the body clock as crucial to improving health and personalizing medicine. This team hopes that by understanding the genetics of sleep, we can gain insights on how to help people during their waking hours.

Abstract: Being a morning person is a behavioural indicator of a person’s underlying circadian rhythm. Using genome-wide data from 697,828 UK Biobank and 23andMe participants we increase the number of genetic loci associated with being a morning person from 24 to 351. Using data from 85,760 individuals with activity-monitor derived measures of sleep timing we find that the chronotype loci associate with sleep timing: the mean sleep timing of the 5% of individuals carrying the most morningness alleles is 25 min earlier than the 5% carrying the fewest. The loci are enriched for genes involved in circadian regulation, cAMP, glutamate and insulin signalling pathways, and those expressed in the retina, hindbrain, hypothalamus, and pituitary. Using Mendelian Randomisation, we show that being a morning person is causally associated with better mental health but does not affect BMI or risk of Type 2 diabetes. This study offers insights into circadian biology and its links to disease in humans.