“Our genetics are set at birth so some of the biases that affect other kinds of epidemiological research tend not to affect genetic studies,” Daghlas said.
Over 340 common genetic variants are known to influence a person’s natural tendency to sleep at a certain time. Researchers analyzed deidentified genetic data on the variants from as many as 850,000 people. Data included 85,000 individuals who had worn wearable sleep trackers for seven days. Also involved were 250,000 people who completed sleep-preference surveys.
The biggest sample showed around one-third of surveyed people self-identified as morning birds, 9% were night owls while everyone else was in the middle. On average the sleep mid-point was 3 a.m. That means people dozed off at 11 p.m. and awoke at 6 a.m.
Scientists also learned that people with genetic variants which influence them to be early risers also have less depression risk. The analysis showed that with every one-hour earlier sleep midpoint there was a 23% decreased risk of major depressive disorder. For example, if someone who usually sleeps at 1 a.m. goes to bed at midnight and sleeps the same time span, they could lower their risk by 23%. If that person has an 11 p.m. bedtime, their risk could be reduced by around 40%.
As for why this is so, research indicates that more light exposure in the daytime leads to a surge of hormonal influences that can affect mood. Other research shows a biological clock that trends differently than most peoples’ alone can be depressing.
“We live in a society that is designed for morning people, and evening people often feel as if they are in a constant state of misalignment with that societal clock,” Daghlas said.
Vetter says that people who want to shift their sleep times should have bright days and dark nights.
“Have your morning coffee on the porch. Walk or ride your bike to work if you can, and dim those electronics in the evening,” she said.
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