When you think of the circadian rhythm, you probably think about sleep. But our bodily rhythms, which also involve things like hormone production and cell regeneration, are our brain’s way of adapting the body for the changes that occur throughout each 24 hour day—like cooling down the body at night for sleeping. And they’re crucial for our wellbeing; research has consistently shown that when these rhythms are disrupted, people are more likely to experience mental health problems.
But according to researchers at the University of Glasgow, this taken-for-granted connection between circadian rhythms and mental health has been backed mostly by studies that used unreliable research methods and too small sample sizes. Until last week, when they published the findings from their own study, the first to objectively measure a sample of more than 91,000 people to confirm the link between a disrupted circadian rhythm and depression.
“Previous studies have looked at this, but have tended to use subjective reports. For example, just asking people their opinion of how stable their circadian rhythms are. Or they’ve used small samples,” said Laura Lyall, the new study’s lead author, in a podcast released last week by The Lancet Psychiatry.
For their study, they identified what's known as a circadian "relative amplitude variable," which tells how much or how little a person’s rest-activity cycle is disrupted. Then, they compared what they found with other measures—including depression, happiness, and cognitive ability.
“We found that people who tended to have lower relative amplitude, indicating that they’ve got less marked distinction between their rest and activity periods, were more likely to meet the criteria for lifetime depression and bipolar disorder,” Lyall says. They also found that people with more circadian rhythm disruptions had lower subjective happiness levels, lower health satisfaction, and were more likely to report feeling lonely. And they had greater mood instability, higher levels of neuroticism, and slower reaction times.
In other words, the stability of your circadian rhythm is intimately linked with not only how you feel emotionally, but also your overall wellbeing. The study results have an important limitation, though. It’s not yet clear whether the disrupted circadian rhythm causes the depression, or if the depression causes the disrupted circadian rhythm.
“These are observational associations and cannot tell us whether mood disorders and reduced wellbeing cause disturbed rest-activity patterns," Lyall says, "or whether disturbed circadian [rhythms] make people vulnerable to mood disorders and poorer wellbeing."
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But the correlations were strong enough to suggest that if your circadian rhythm is off-kilter—if you go to bed at radically different times from one night to the next, for example—that you are more likely to experience depression. Still, more research is needed to determine whether a person has depression because they aren’t sleeping well, or they aren’t sleeping well because they have depression. A better understanding of this relationship would inform the focus of interventions to reduce mental illness and other health problems associated with circadian rhythm.
“It’s hard to disentangle the causality of these things. You can observe the correlations but you’d like to know which thing you should try to change,” says David Hinds, a geneticist who has studied sleep habits at Silicon Valley genomics company 23andMe. “If the causal direction is going the wrong way, you can change the behavior but it’s not actually going to affect the outcome.”
Hinds says looking at the genetic component of circadian rhythms and mental illness may provide some answers. “One way to determine the direction of cause and effect for these things is to look at the genetics,” he says, “because you know the genetics can’t be an effect of something else. It has to be on the cause side.”
There could be some clues in the study of non-human animals as well. Ilia Karatsoreos, a professor of neuroscience at Washington State University, has written at length about the links between circadian rhythms and psychiatric disease. He thinks the best data we have actually comes from the study of non-human animals.
“There is work that shows disrupting the cellular clock—or, the molecular ‘gears’ of the circadian clock—can increase depressive-like and manic-like behaviors, as well as cognitive function in mice and rats,” Karatsoreos says. “Similarly, restricting sleep in non-human animal models can lead to changes in affective behaviors and cognition.”
The science of circadian rhythms is incredibly complex even if we simplify it by focusing on just one aspect, like sleep. But a sleep schedule may be one of the simpler avenues for keeping the internal clocks in check. One thing we can do, Karatsoreos says, is avoid "social jet lag"—that thing you experience when you have to be up early all week and then sleep late on the weekends, only to repeat the cycle again each week.
“While it may seem like a good thing to try to ‘catch up' on lost sleep over the weekends, you’re basically flying from New York to LA on Friday, and then flying back on Monday morning every single week,” Karatsoreos says. “This causes your brain clock and sleep rhythms to become more desynchronized from the environment and is likely a negative thing over the long run.”
To offset the potentially negative impacts of radically different sleeping patterns from the workdays to the weekend, Karatsoreos suggests maintaining as much consistency in your sleep schedule throughout the week as you can.
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