Waking up just an hour earlier could reduce a person’s risk of major depression by 23%, a powerful new genetic study suggests.
The study of 840,000 people, conducted by researchers at the University of Colorado Boulder and the Broad Institute at MIT and Harvard, represents some of the strongest evidence yet that chronotype – a person’s propensity to sleep at a given time ) influences the risk of depression.
It is also one of the first studies to quantify how much or little change is required to influence mental health.
As people emerge from working and attending school remotely after the pandemic, a trend that has led many to switch to a later sleep schedule, the findings could have important implications.
“We have known for some time that there is a relationship between sleep time and mood, but a question we often hear from doctors is: how soon do we need to change people to see a benefit?” said lead author Celine Vetter, assistant professor of integrative physiology at CU Boulder. “We found that even an hour before bedtime is associated with a significantly lower risk of depression.”
Previous observational studies have shown that night owls are twice as likely to suffer from depression as early risers, regardless of how long they sleep. But because mood disorders themselves can disrupt sleep patterns, researchers have had a hard time figuring out what causes what.
Other studies had small sample sizes, relied on single time point questionnaires, or did not take into account environmental factors that can influence both sleep timing and mood, potentially confusing the results.
In 2018, Vetter published a large long-term study of 32,000 nurses showing that “early risers” were up to 27% less likely to develop depression over the course of four years, but that raised the question: What does it mean to be an early riser?
To get a clearer idea of whether changing sleep time earlier is really protective and how much change is required, lead author Iyas Daghlas, MD, turned to data from DNA testing company 23 and Me and the database of biomedical data UK Biobank. Daghlas then used a method called “Mendelian randomization” that takes advantage of genetic associations to help decipher cause and effect.
“Our genetics are established at birth, so some of the biases that affect other types of epidemiological research tend not to affect genetic studies,” said Daghlas, who graduated in May from Harvard Medical School.
More than 340 common genetic variants, including variants of the so-called PER2 “clock gene,” are known to influence a person’s chronotype, with genetics collectively explaining between 12% and 42% of our preference for sleep time .
The researchers evaluated unidentified genetic data on these variants from up to 850,000 people, including data from 85,000 who had used portable sleep trackers for 7 days and 250,000 who had filled out sleep preference questionnaires. This gave them a more detailed picture, so far, of how gene variants influence when we sleep and wake up, the results of which were just published in the journal. JAMA Psychiatry.
In the largest of these samples, about a third of the subjects surveyed self-identified as morning larks, 9% were night owls, and the rest were in the middle. In general, the average sleep midpoint was 3 a.m., which means they went to bed at 11 p.m. and woke up at 6 a.m.
With this information in hand, the researchers drew on a different sample that included genetic information along with anonymous prescription and medical records and surveys of major depressive disorder diagnoses.
Using novel statistical techniques, they asked: Are those with genetic variants that predispose them to being early risers also have a lower risk of depression?
The answer is a firm yes.
Each midpoint of sleep an hour earlier (midway between bedtime and waking up) corresponded to a 23% lower risk of major depressive disorder.
This suggests that if someone who normally goes to bed at 1 am goes to bed at midnight and sleeps the same amount of time, they could reduce their risk by 23%; if they go to bed at 11pm, they could reduce it by about 40%.
It’s unclear from the study whether those who are already early risers could benefit from waking up even earlier. But for those in the mid-range or late-night range, it would probably be helpful to switch to an earlier bedtime.
What could explain this effect?
Some research suggests that getting more exposure to light during the day, which early risers often have, results in a cascade of hormonal impacts that can influence mood.
Others point out that having a body clock, or circadian rhythm, that has different tendencies than most people can be depressing in and of itself.
“We live in a society that is designed for people in the morning, and people in the evening often feel like they’re in a constant state of misalignment with that social clock,” Daghlas said.
He emphasizes that a large randomized clinical trial is necessary to definitively determine whether going to bed early can reduce depression. “But this study definitely shifts the weight of evidence toward supporting a causal effect of sleep time on depression.”
For those who want to switch to an earlier sleep schedule, Vetter offers this advice:
“Keep your days bright and your nights dark,” she says. Have your morning coffee on the porch. Walk or bike to work if you can, and dim those electronic devices at night. “