Getting in sync: Wearables reveal happiest times to sleep

A new study from the University of Michigan shows that when people’s sleep cycles don’t align with their biological clocks, or circadian rhythms, it can have a dramatic impact on their mood.

However, conversely, this means that sleeping when the body expects it can effectively boost a person’s emotional state and may reduce symptoms associated with mood disorders, said senior author Daniel Fogg.

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

“But this is the key factor that we can actually control. We can’t control the events of someone’s life. We can’t control their relationships or their genes. But what we can do is look very carefully at their individual sleep patterns and day and night Rhythm really understands how this affects their mood.

The research is published in the journal npj digital medicine.

Interns, Fitbits and questionnaires

Sleep has long been known to affect mood, but mostly in a conceptual, almost light-hearted way. For example, when discussing this connection, we often use words like “cranky” or “critical.”

Yet previous research has consistently found a link between sleep (its duration, quality and disruption) and serious mental health problems, including suicide risk.

“Sleep is important to us, but maybe not as important as we care about depression,” Fogg said. “But a lot of research shows that mood affects circadian rhythms and sleep, and that circadian rhythms and sleep affect mood.”

However, the study was conducted almost entirely in a controlled environment, Fogg said. So he and his team set out to find these effects in the real world, and opportunities to harness them to improve mood.

The program is made possible in part by the Intern Health Study, a program funded by the National Institutes of Health at the University of Michigan that works with hundreds of first-year physicians in training. As part of the study, interns completed daily mood surveys while wearing fitness trackers (i.e., Fitbits) to monitor their heart rate, activity, and sleep habits. This research was also supported by the National Science Foundation.

Forger and his team developed algorithms to evaluate Fitbit data and extract quantitative information about people’s circadian rhythms, sleep cycles, and how well those rhythms are coordinated. By combining this with the Intern Health Study’s daily mood survey and using a quarterly depression screening questionnaire, the team was able to draw links between these concordances and real-world indicators of mental health.

Information from the questionnaire—the nine-item Patient Health Questionnaire (PHQ-9), which is widely used in research and clinical settings—revealed an especially striking number when it came to people whose rhythms were out of sync.

“When people start to become desynchronized, we see the PHQ-9 rise by an average of 2.5 points,” Forger said. “This is clinically important.”

But exactly what the misalignment is is also important, says Minki Lee, one of the study’s lead authors.

“It’s not just ‘if you go to bed earlier, you’ll be happier,'” said Lee, an undergraduate researcher and 2023 Goldwater Scholar. “To some extent, this is true, but it’s because your sleep schedule aligns with your internal rhythms.”

the rhythms of our bodies

The team was able to extract distinct features, or biomarkers, of three different important patterns.

There is a central circadian clock, which keeps time in the suprachiasmatic nucleus of the brain. It also coordinates peripheral circadian clocks in other parts of the body. In the study, the team analyzed the heart’s peripheral clock.

Forger says that in a typical person, the heart knows it needs to be ready and is more active at 2 p.m. than at 2 a.m. thanks to its peripheral clock.

The final pattern the team could measure was the interns’ sleep cycles.

The research team found that, in general, sleep cycles that are out of sync with your peripheral body clock (what your heart thinks is time) can have a negative impact on mood.

However, when a person’s central circadian rhythm is out of balance with sleep cycles, negative effects can occur when interns work shifts. That is, the misalignment between their sleep and their central circadian clock is caused by their occupation.

When this mismatch affects emotions, the effects are more pronounced than in the case of peripheral mismatches.

Dae Wook Kim, another lead author of the study, said: “Specifically, misalignment between the central circadian clock and sleep showed the strongest negative association with mood and depression symptoms, including poor sleep, appetite problems and even suicidal thoughts. “

Kim helped conduct the research as a postdoctoral researcher at the University of Michigan and is now an assistant professor at the Korea Advanced Institute of Science and Technology.

“These findings challenge previous assumptions about the uniform effects of circadian disruption of different circadian clocks,” King said.

Challenging these assumptions raises new questions about how and when these disturbances manifest themselves in other populations, including students, older adults and people diagnosed with mental illness, King said. The team has already begun bringing its research methods to some of these groups.

“This suggests that we have to consider the different rhythms that represent different parts of the body and consider them based on your working conditions and overall lifestyle,” Lee said.

Researchers say it’s not surprising that context matters. After all, students cram for exams and vacationers travel halfway around the world without late nights or jet lag weighing heavily on their moods.

But research shows that we understand when these distractions are affecting us and when to take a break to remedy the problem with technology at our fingertips. Or, to be more precise, on our wrists.

“That’s why it’s scalable,” Fogg said. “That’s why I think this can help a lot of people.”