Getting in sync: Wearables reveal happiest times to sleep

(Press-News.org) Sleep schedules are often one of the first things that people choose to compromise in order to check everything off their to-do lists, especially with the end of the year approaching. But folks hoping for happy holidays should reconsider. 

A new study from the University of Michigan shows that when people's sleep cycles are misaligned with their internal clocks, or circadian rhythms, it can have drastic effects on their moods.

Conversely, however, that means getting sleep when the body's expecting it provides a potent boost to one's emotional state and could alleviate symptoms associated with mood disorders, said senior author Daniel Forger.

"This is not going to solve depression. We need to be very, very clear about that," said Forger, professor in the Department of Mathematics and director of the Michigan Center for Applied and Interdisciplinary Mathematics.

"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."

The research is published in the journal npj Digital Medicine.

Interns, Fitbits and questionnaires People have long known that sleep affects mood, but mostly in a conceptual, almost lighthearted way. For instance, we often use words like "cranky" or "fussy" when discussing this connection.

Yet previous studies have consistently found links between sleep—its duration, quality and disruption—and serious mental health concerns, including suicide risk.

"Sleep is important to us, but maybe not in the same way we care about depression," Forger said. "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."

This research, however, has almost exclusively been performed in controlled settings, Forger said. So he and his team set out to find these effects—and opportunities to use them to improve moods—in the real world.

This project was made possible, in part, by the Intern Health Study, a project funded by the National Institutes of Health at the University of Michigan which works with hundreds of first-year training physicians. As part of the study, the interns complete routine mood surveys while wearing fitness trackers—namely, Fitbits—that monitor their heart rate, activity and sleeping habits. This study was also supported by the National Science Foundation.

Forger and his team have developed algorithms to assess Fitbit data and extract quantitative information about people's circadian rhythms, their sleep cycles and how well those align. By coupling that with the Intern Health Study's daily mood surveys and also using quarterly depression screening questionnaires, the team could establish links between those alignments and real-world measures of mental health.

The information from the questionnaire—the nine-item Patient Health Questionnaire, or PHQ-9, which is widely used in research and clinics—yielded a particularly striking figure when it came to people with desynchronized rhythms.

"When people start to get desynchronized, we see the PHQ-9 go up, on average, by 2.5," Forger said. "That’s clinically important."

But what exactly is misaligned also matters, said one of the study's lead authors, Minki Lee.

"It's not just, 'If you go to bed earlier, you will be happier,'" said Lee, who is an undergraduate researcher and a 2023 Goldwater Scholar. "To some degree, that will be true, but it will be because your sleep schedule is aligning with your internal rhythms.'"

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 found that, generally speaking, having a sleep cycle out of sync with the peripheral circadian clock—that is, what time your heart thought it was—had a negative effect on mood.

When a person's central circadian rhythm was out of whack with respect to their sleep cycle, however, a negative effect was seen when an intern was doing shift work. That is, the misalignment between their sleep and central internal clock was driven by their occupation.

And when this mismatch was affecting mood, its effect was more pronounced than in the peripheral mismatch case.

"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," said Dae Wook Kim, another lead author of the study. 

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.

"These findings challenge prior assumptions about the uniform impact of circadian disruptions across different physiological clocks," Kim said.

Challenging these assumptions opens up new questions about how and when these disruptions manifest in other groups of people, including students, older adults and individuals diagnosed with psychiatric disorders, Kim said. The team is already starting to bring its study methodology to some of those groups.

"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," Lee said.

It's not surprising that context matters, the researchers said. 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.

But the study shows we understand when these disruptions are impacting us and when getting some rest can remedy that using technology at our fingertips. Or, more accurately, on our wrists.

"That's why this is scalable," Forger said. "That's why I think this could help tons of people."

 

END