Some people jump out of bed at the crack of dawn, while for others, early morning is only something experienced if they’ve stayed up the night before.
But night owls take note: people who are genetically programmed to be early risers have a lower risk of schizophrenia or depression, according to a large new study.
“We show that being a morning person is causally associated with better mental health,” an international team of researchers reported today in Nature Communications.
The genetic sweep of nearly 700,000 people identified more than 320 new genes on top of 24 genes already known to influence whether someone is a lark or a night owl.
People with the most genes identified were likely to rise and shine 25 minutes earlier than those with the least.
Whether you’re an early or a late riser is driven by your biological clock, but very little is known about how your circadian rhythms influence disease.
Previous studies have suggested night owls are more likely to be obese or have type 2 diabetes or depression, but it was unclear whether these conditions themselves affected when people slept or were due to other factors such as exercise is less common during the night.
This is the first large-scale study to probe the causal link between circadian rhythms and health disorders, said co-author Enda Byrne of Queensland University.
“People have studied circadian rhythms in the context of psychiatric disorders for a number of years looking one gene at a time to get an insight into it,” Dr Byrne said.
“But this is sort of quite a big breakthrough of looking through the whole genome to find this link.”
Circadian rhythm genes discovered
The researchers analysed genetic variations of people who participated in the UK Biobank study and from private US genetics company 23andMe.
The participants contributing to each database were asked to indicate whether they were a morning or night person to see which genes they had in common.
Along with genes related to the regulation of circadian rhythms and insulin, there were genes associated with retinal cells in the eye, which pick up light signals and send them to the brain.
“Light is the main environmental influence that helps us to synch our rhythms,” Dr Byrne said.
Because self-reporting can be inaccurate, the researchers also analysed a subset of more than 85,000 people who wore activity monitors to get an objective measure of body clock behaviour.
“We got a feel for what time people actually woke up at, and what time they actually went to bed at,” Dr Byrne said.
This showed there was no relationship between when people went to bed and sleep quality or duration.
“It wasn’t that morning people were going to bed early so they were getting more sleep than evening types which was making them feel better,” Dr Byrne said.
Surprisingly, genetically predisposed evening types were no more likely to be at risk of obesity or type 2 diabetes than early risers.
There was, however, a small but statistically significant correlation between genetic variants that increased the chances of being a night owl and an increased risk of mental health conditions.
Dr Byrne said more work was needed to understand the link.
“For instance, it could be that evening types are forced to get up early which gets them out of their natural rhythms because of a job, or kids, or some other environmental influence in their lives,” he said.
“That combination of genetic predisposition with the environmental factors could be what’s leading to more symptoms of depression.”
Implications for night owls
Ian Hickie of the Brain and Mind Centre at the University of Sydney said the genetic study backed up what psychiatrists had long thought: that mood disorders were linked to circadian rhythms, not sleep disorders.
He said this explained why 20 years of research had failed to find sleep genes linked to mood disorders.
“They’ve finally cracked the nut by saying ‘you know what, we were looking at the wrong thing’,” said Professor Hickie, who was not involved in the study.
“It’s not sleep, it’s the body clock.”
Professor Hickie said the body clock had a wake and sleep period and everyone had a normal set point, known as ‘morningness’.
While people with more morning genes had a body clock that fitted well with a 9 to 5 routine, night owls didn’t.
Being an evening person did not mean you were poorly motivated, Professor Hickie said.
“There is an idea that morning people are all good, hardworking people, but they’re just genetically set that way,” he said.
But being a night owl did mean your body clock could more easily be thrown out, and this could increase your susceptibility to mental illness.
While it was not possible to “become a morning person” Professor Hickie said night owls could take steps to regulate their daily cycle to reduce the risk of developing mental illness.
“Night people need to be very careful … they can very easily stay up all night.”
Professor Hickie said the findings could lead to more effective mental health treatments based on using things such as light, exercise and melatonin-based drugs instead of serotonin-based drugs used to treat sleep disorders.
“Now the great hope, of course, in the future is if you had a genetic test before you choose the right drug or behavioural treatment,” said Professor Hickie, adding that this is the focus of major studies of depression and bipolar currently underway in Australia.