Wearables reveal the happiest sleep times

Sleep schedules are often one of the first things people choose to compromise to check everything off their to-do lists, especially as the end of the year approaches. But people hoping for a nice holiday should reconsider.

A new study from the University of Michigan shows that when people’s sleep cycles don’t align properly with their internal clock or circadian rhythm, it can have drastic effects on their mood.

Conversely, however, this means sleeping when the body expects it to provide a powerful boost to emotional states and can alleviate symptoms associated with mood disorders, according to senior author Daniel Forger.

“This is not going to solve the depression. We have 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 look very carefully at their individual sleep patterns and circadian rhythms.” to really see how that affects their mood.”

The research is published in npj Digital medicine.

Interns, Fitbits and questionnaires

People have long known that sleep affects mood, but usually in a conceptual, almost light-hearted way. For example, we often use words like “grumpy” or “fussy” when discussing this connection.

Yet previous studies have consistently found links between sleep – its duration, quality and disruption – and serious mental health problems, including the risk of suicide.

“Sleep is important to us, but maybe not in the same way that we care about depression,” Forger said. “But a tremendous amount of research has emerged showing that mood influences circadian rhythms and sleep, and that circadian rhythms and sleep influence mood.”

However, this research has been conducted almost exclusively in controlled environments, Forger said. So he and his team set out to find these effects – and ways to use them to improve mood – in the real world.

This project was made possible by the Internal Health Study, which collaborates with hundreds of first-year doctors in training. As part of the study, the interns complete routine mood surveys while wearing fitness trackers, namely Fitbits, which monitor their heart rate, activity and sleep habits.

Forger and his team have developed algorithms to assess Fitbit data and obtain quantitative information about people’s circadian rhythms, their sleep cycles, and how well they align. By linking that to the Internal Health Study’s daily mood surveys and using quarterly depression screening questionnaires, the team was able to make connections between these 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 – produced a particularly striking figure when it came to people with desynchronized rhythms.

“When people become desynchronized, we see the PHQ-9 increase by an average of 2.5,” Forger said. “That is clinically important.”

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

“It’s not just, ‘If you go to bed earlier, you’ll be happier,’” says Lee, an undergraduate researcher and a 2023 Goldwater Scholar. “To some extent that will be true, but it will be because your sleep schedule is in line with your internal rhythms.”

The rhythm of our body

The team was able to extract telling features, or biomarkers, of three different key 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 their study, the team analyzed the peripheral clock in the heart.

For a typical person, the heart knows to be more active at 2 p.m. than at 2 a.m., thanks to the peripheral clock, Forger said.

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

The team found that in general, a sleep cycle that was out of sync with the peripheral circadian clock (that is, what time your heart thought it was) had a negative effect on mood.

However, when a person’s central circadian rhythm was out of sync with their sleep cycle, a negative effect was observed when an intern worked shifts. That is, the misalignment between their sleep and the central internal clock was caused by their activity.

And when this mismatch affected mood, its effect was greater than in the case of the peripheral mismatch.

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

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

“These findings challenge previous assumptions about the uniform impact of circadian disruptions on different physiological clocks,” Kim said.

Challenging these assumptions opens 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 present its research methodology to some of these groups.

“This shows us that we need to look at different rhythms that represent different parts of your body and consider them in light of your working conditions and your lifestyle in general,” Lee said.

Not surprisingly, context matters, the researchers said. After all, students are cramming for exams and vacationers travel halfway around the world without letting late nights or jet lag significantly affect their mood.

But the research shows that we understand when these disruptions are impacting us and when getting some rest can remedy them with the help of technology at our fingertips. Or rather: on our wrists.

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