A recent study from the University of Michigan reveals that when people’s sleep patterns do not align with their natural internal clocks, also known as circadian rhythms, it can significantly impact their mood.
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Sleep is often one of the first things people sacrifice in order to tick items off their to-do lists, particularly as the year draws to a close. However, those aiming for a joyful holiday season may want to reconsider this choice.
On the flip side, sleeping in sync with the body’s natural rhythms can enhance emotional well-being and potentially reduce symptoms linked to mood disorders, according to senior author Daniel Forger.
This is not going to solve depression. We need to be very, very clear about that. But this is a key factor that we can actually control. We can’t control someone’s life events. We can’t control their relationships or their genetics. But what we can do is very carefully look at their individual sleep patterns and circadian rhythms to really see how that’s affecting their mood.
Daniel Forger, Professor, Department of Mathematics, University of Michigan
Interns, Fitbits and Questionnaires
While the connection between sleep and mood has long been acknowledged, it has often been seen in a more general sense. People commonly describe the relationship with terms like “cranky” or “fussy.” Yet past research has repeatedly shown strong links between sleep—its length, quality, and disruptions—and serious mental health concerns, including suicide risk.
Forger added, “Sleep is important to us, but maybe not in the same way we care about depression. But there’s been a tremendous amount of research coming out showing that mood affects circadian rhythms and sleep, and that circadian rhythms and sleep affect mood.”
However, most of this research has been conducted in controlled environments, Forger pointed out. His team aimed to explore these effects in real-world scenarios and identify ways to leverage them to improve mood.
This project was made possible in part by the Intern Health Study, an initiative funded by the National Institutes of Health at the University of Michigan. The study involves hundreds of first-year physicians, who complete daily mood surveys and wear fitness trackers—Fitbits—that monitor their heart rate, activity, and sleep. The study also received support from the National Science Foundation.
Forger and his team created algorithms to analyze Fitbit data, extracting detailed information about participants’ circadian rhythms, sleep cycles, and their alignment. By pairing this data with daily mood surveys and quarterly depression screening using the widely recognized PHQ-9 questionnaire, they were able to draw connections between sleep patterns and mental health.
The PHQ-9 responses revealed an important trend among participants with desynchronized rhythms.
“When people start to get desynchronized, we see the PHQ-9 go up, on average, by 2.5. That is clinically important,” Forger added.
However, the specifics of misalignment also matter explained lead author Minki Lee.
It is not just, ‘If you go to bed earlier, you will be happier. To some degree, that will be true, but it will be because your sleep schedule is aligning with your internal rhythms.
Minki Lee, Study Lead Author and Undergraduate Researcher, University of Michigan
The Rhythms of Our Bodies
The team was able to extract telling features, or biomarkers, of three different important patterns.
There was the central circadian clock, which keeps time in the suprachiasmatic nuclei of the brain. It also coordinates peripheral circadian clocks in other parts of the body. In its study, the team analyzed the peripheral clock in the heart.
For a typical person, the heart knows that it needs to be ready to be more active at 2 p.m. than at 2 a.m. thanks to its peripheral clock, Forger said.
The final pattern the team could measure was the interns’ sleep cycles.
The team discovered that, in general, a misaligned sleep cycle with the peripheral circadian clock—meaning the heart's perception of time—had a negative impact on mood.
However, when the central circadian rhythm was out of sync with the sleep cycle, a more significant effect on mood was observed, particularly in interns working shift schedules. In this case, the misalignment between their sleep patterns and central internal clock was largely influenced by their work.
The impact on mood was found to be more pronounced when the central circadian rhythm was disrupted compared to when there was only a mismatch with the peripheral clock.
Specifically, the misalignment between the central circadian clock and sleep exhibited the strongest negative association with mood and depressive symptoms, including poor sleep, appetite issues, and even suicidal thoughts.
Dae Wook Kim, Study Lead Author and Assistant Professor, Department of Brain & Cognitive Sciences, Korea Advanced Institute of Science and Technology
Kim helped conduct the study as a postdoctoral fellow at U-M and is now an assistant professor at the Korea Advanced Institute of Science and Technology.
He added, “These findings challenge prior assumptions about the uniform impact of circadian disruptions across different physiological clocks.”
Challenging these beliefs raises new concerns about how and when these disturbances occur in other groups of people, such as students, older adults, and those with psychiatric problems, Kim said. The team is already beginning to apply their research methods to some of these groups.
Lee added, “This shows us we have to look at different rhythms representing different parts of your body and consider them in light of your working conditions and your lifestyle in general.”
According to the researchers, it is no surprise that context is important. After all, students cram for exams and vacationers travel halfway around the world without having the all-nighters or jet lag significantly impair their moods.
However, the study did demonstrate that we can recognize when these disruptions are affecting us and when rest can help, thanks to the technology available at our fingertips—or more precisely, on our wrists.
“That is why this is scalable. That is why I think this could help tons of people,” Forger concluded.
Journal Reference:
Lee, M. P., et. al. (2024) The real-world association between digital markers of circadian disruption and mental health risks. npj Digital Medicine. doi.org/10.1038/s41746-024-01348-6