Waking up just one hour earlier could reduce a person's risk of major depression by 23 per cent, suggests a sweeping new genetic study.
The study indicates that if someone who normally goes to bed at 1 a.m. goes to bed at midnight instead and sleeps the same duration, they could cut their risk by 23 per cent; if they go to bed at 11 p.m., they could cut it by about 40 per cent.
"We found that even one-hour earlier sleep timing is associated with significantly lower risk of depression," said researcher Celine Vetter from the University Of Colorado At Boulder.
Some research suggests that getting greater light exposure during the day, which early-risers tend to get, results in a cascade of hormonal impacts that can influence mood.
Others note that having a biological clock, or circadian rhythm, that trends differently than most peoples' can in itself be depressing.
For the study, published in the journal JAMA Psychiatry, the team turned to data from a DNA testing company and the biomedical database UK Biobank. They then used a method called "Mendelian randomisation" that leverages genetic associations to help decipher cause and effect.
More than 340 common genetic variants, including variants in the so-called "clock gene" PER2, are known to influence a person's chronotype, and genetics collectively explains 12-42 per cent of our sleep timing preference.
The researchers assessed de-identified genetic data on these variants from up to 850,000 individuals, including data from 85,000 who had worn wearable sleep trackers for 7 days and 250,000 who had filled out sleep-preference questionnaires.
In the largest of these samples, about a third of surveyed subjects self-identified as morning larks, 9 per cent were night owls and the rest were in the middle. Overall, the average sleep mid-point was 3 a.m., meaning they went to bed at 11 p.m. and got up at 6 a.m.
With this information in hand, the researchers turned to a different sample which included genetic information along with anonymised medical and prescription records and surveys about diagnoses of major depressive disorder.
Using novel statistical techniques, they asked: Do those with genetic variants which predispose them to be early risers also have a lower risk of depression? The answer is a firm yes, the study noted.