Research reveals links between mood and disruptions in circadian rhythms based on Fitbit data from numerous medical interns.
As the year wraps up, many people often sacrifice their sleep to finish their tasks. Those looking forward to joyful holidays should think twice about this habit.
A recent study from the University of Michigan indicates that when individuals’ sleep patterns are out of sync with their internal biological clocks, also known as circadian rhythms, it significantly impacts their emotional well-being.
On the flip side, aligning sleep with the body’s expectations can enhance mood and help alleviate signs of mood disorders, according to senior author Daniel Forger.
“We want to be clear, this won’t cure depression,” said Forger, a professor in the Department of Mathematics and director of the Michigan Center for Applied and Interdisciplinary Mathematics.
“However, sleep is a modifiable aspect of our lives. While we can’t control external factors like life events, relationships, or genetic predispositions, we can closely examine individual sleep habits and circadian rhythms to understand their effects on mood better.”
The findings have been published in the journal npj Digital Medicine.
Interns, Fitbits, and Surveys
It is commonly understood that sleep influences mood in a broad, often casual way, typically expressed with terms like “irritable” or “grumpy.”
However, earlier research has established clear connections between various sleep attributes—such as duration, quality, and disruption—and more severe mental health challenges, including the risk of suicide.
“While many recognize the importance of sleep, it doesn’t always receive the same attention as depression,” Forger noted. “But there’s a growing body of research indicating that mood can impact circadian rhythms and sleep, and likewise, that circadian rhythms and sleep can affect mood.”
Previous studies have mainly taken place in controlled environments. Hence, Forger and his team aimed to explore these effects and their potential for mood enhancement in everyday settings.
This research was facilitated by the Intern Health Study at the University of Michigan, which is funded by the National Institutes of Health and involves numerous first-year medical interns. These interns regularly participate in mood surveys while using fitness trackers—specifically Fitbits—that monitor their heart rate, activity levels, and sleep patterns. This project also received backing from the National Science Foundation.
Forger’s team has created algorithms to analyze Fitbit data, allowing them to glean quantitative insights into individuals’ circadian rhythms, sleep patterns, and how well these align. By integrating this data with the interns’ daily mood evaluations and seasonal depression screenings, the researchers could draw connections between these sleep alignments and real-life mental health metrics.
The mood assessment results, derived from the nine-question Patient Health Questionnaire (PHQ-9), a widely recognized tool in research and clinical settings, revealed a significant finding regarding those with misaligned rhythms.
“We observed that as people’s rhythms become desynchronized, their PHQ-9 scores increase by an average of 2.5,” Forger remarked. “This is a clinically relevant change.”
However, what constitutes a misalignment is also crucial, emphasized study co-author Minki Lee.
“It’s not simply a matter of ‘going to bed earlier means you’ll feel happier,'” said Lee, an undergraduate researcher and a 2023 Goldwater Scholar. “While that may hold some truth, it’s essentially about realigning your sleep pattern with your internal rhythms.”
The Body’s Rhythms
The team successfully identified essential markers, or indicators, of three significant rhythmic patterns.
First is the central circadian clock that manages time in the brain’s suprachiasmatic nuclei and coordinates peripheral clocks in other body regions. In this study, the team assessed the peripheral clock specific to the heart.
Typically, a person’s heart is primed for activity during specific times, for instance being more ready for action at 2 p.m. than at 2 a.m., due to its internal clock, Forger explained.
The last notable pattern examined was the interns’ sleep cycles.
The researchers found that generally, when sleep cycles were out of sync with the peripheral circadian clock (essentially, when the heart’s timing differs), it negatively affected mood.
However, when discrepancies occurred between one’s central circadian rhythm and sleep, more pronounced negative effects were observed, particularly among interns engaged in shift work. This misalignment was largely driven by their work schedules.
Moreover, disruptions affecting mood were more acute in the case of central rhythm misalignment compared to peripheral clock issues.
“Specifically, the discord between the central circadian clock and sleep showed the highest negative correlation with mood and depressive symptoms, including sleep disturbances, appetite challenges, and even suicidal thoughts,” stated Dae Wook Kim, another lead author and former postdoctoral fellow at U-M, who is now an assistant professor at the Korea Advanced Institute of Science and Technology.
“These findings redefine prior beliefs about the uniform impacts of circadian disruptions across various physiological clocks,” Kim remarked.
By challenging these existing beliefs, new inquiries arise surrounding how and when these disruptions occur in other demographics, such as students, older adults, and individuals with psychiatric conditions. The research team is already beginning to apply its methodology to these groups.
“We must focus on different rhythms that represent various bodily functions and assess them in relation to one’s work conditions and overall lifestyle,” Lee added.
It’s understandable that context is significant, the researchers stated. After all, students may endure late-night study sessions, and travelers often overcome jet lag without it adversely impacting their moods to a large extent.
This study reveals an awareness of how these disruptions affect individuals and the potential benefit of adequate rest, aided by accessible technology—essentially, what’s on our wrists.
“That’s why this research has scalable potential,” Forger noted. “I believe it could benefit a vast number of people.”
