The study showed that if someone who usually fell asleep at 1 a.m. went to bed in the middle of the night and slept the same amount of time, they could reduce their risk by 23 percent; If they went to bed at 11 p.m., they could cut it by about 40 percent.
“We found that an hour before bedtime was associated with a significantly lower risk of depression,” said researcher Celine Vetter of the University of Colorado at Boulder.
Several studies have shown that greater light exposure during the day, which tends to wake up earlier, triggers a series of hormonal effects that can affect mood.
Others note that having a biological clock, or circadian rhythm, with a different head than most people can be frustrating on its own.
For the study, published in the journal JAMA Psychiatry, the team turned to data from DNA testing companies and UK Biobank. Then they used a method called “Mendelian randomization,” which strengthens genetic associations to help decode 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 temporal patterns, and these genes collectively explain 12-42 percent of our sleep preferences.
The researchers evaluated non-specific genetic data on variants of up to 850,000 individuals, including data from 85,000 who took a sleep tracker for 7 days and 250,000 who filled out a sleep preference questionnaire.
In this largest sample, about one-third of the people surveyed identified themselves as morning larks, 9 percent were nocturnal owls and the rest were in the middle. In general, the average midnight bedtime is 3am, which means they go to bed at 11pm and wake up at 6am.
As this information became available, the researchers turned to different samples that included genetic information along with medical records, prescriptions from anonymous sources, and surveys to diagnose major depressive disorder.
Using a new statistical technique, they wondered: Do those with the genetic variant that make them wake up earlier also have a lower risk of developing depression? The study shows that the answer is yes.
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