A study of 840,000 people by University of Colorado Boulder researchers and researchers at MIT and Harvard provides some of the strongest evidence yet that individual chronotype, the time in which someone tends to go to sleep, influences depression risk. It’s also among the first studies to quantify just how much, or how little, change is needed to influence mental health. Following the pandemic, people are no longer working or attending school remotely, leading to a shift in sleep schedules. The findings may have important implications.
A common question we hear from clinicians is, ‘How early can we shift people to increase their benefits from sleep?’, said senior author Celine Vetter, assistant professor of integrative physiology at CU Boulder. ‘We found that even one hour earlier sleep is associated with a significantly lower risk of depression’. Night owls are twice as likely to suffer from depression as early risers, regardless of how long they sleep. Despite this, sleep disorders themselves can disrupt sleep patterns, so researchers have had a hard time deciphering what causes what. Several other studies have relied on small sample sizes or questionnaires from a single time point, and neglected to take into account environmental factors, which may also affect sleep timing and mood.
It was reported in 2018 by Vetter that ‘early risers’ were 27% less likely to develop depression over four years. However, that begged the question, what exactly constitutes an early riser? To determine whether being able to shift sleep time earlier is truly protective of heart health, the lead author, Dr. Iyas Daghlas turned to data from DNA testing company 23 and Me and the UK Biobank database. Daghlas then used a method known as ‘Mendelian randomization’ to find the cause and effect.
‘Because genetics are set at birth, some of the biases that affect other types of epidemiological research do not apply to genetic studies,’ said Daghlas, who graduated from Harvard Medical School in May. More than 340 common genetic variants, including variants in the so-called ‘clock gene’ PER2, influence a person’s chronotype, and genetics explains 12-42% of our sleep preference. In addition to de-identified data from 850,000 individuals, the researchers also studied sleep-preference questionnaires from 250,000 individuals, 85,000 of whom wore sleep trackers for 7 days. This gave them a more precise picture of how genetic variants affect when we sleep and wake up, even down to the hour.
Approximately a third of the surveyed groups were morning larks, 9% were night owls, and the rest were a mix. The average sleep mid-point was 3 a.m., so they went to bed at 11 p.m. and got up at 6 a.m. In addition to genetic information, the researchers analyzed anonymized medical and prescription records, as well as surveys about diagnoses of major depression. New statistical techniques were used to determine: Are those with genetic variants that predispose them to be early risers also at lower risk of depression? Definitely yes.
Each hour earlier sleep midpoint (halfway between bedtime and wake time) corresponded with a 23% lower risk of major depression. If someone usually goes to bed at 1 a.m. goes to sleep at midnight instead and sleeps the same amount of time, they can reduce their risk by 23%; if they go to bed at 11 p.m., their risk can be reduced by about 40%. The study doesn’t indicate whether early risers could benefit from waking up even earlier. For those in the intermediate or evening range, shifting to an earlier bedtime would probably be helpful.
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Getting more light exposure during the day, which early risers tend to get, can produce a cascade of hormonal effects that influence mood. Others say that having a biological clock that trends differently than most people’s can be depressing. ‘We live in a society that is designed for morning people, and evening people often feel misaligned with that clock’, said Daghlas.
To determine definitively whether going to bed early can reduce depression, a large randomized clinical trial is needed. For those looking to start an earlier sleep schedule, Vetter offers this advice: ‘Keep your days bright and your nights dark,’ she says. ‘Enjoy your morning coffee on the porch. Walk if you can, or ride a bike to work. Dim the lights at night.’
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