Sleep timing has a powerful effect on our mood

Sleep schedules are often disrupted when people rush to complete their end-of-year tasks. But those looking forward to a cheerful holiday season might want to reconsider. 

A new study from the University of Michigan (U-M) suggests that when sleep patterns fall out of sync with the body’s internal clock, it can have dramatic effects on mood.

Conversely, getting rest aligned with our circadian rhythms can deliver a powerful emotional boost and potentially lessen the burden of mood disorder symptoms, said senior author Daniel Forger.

“This is not going to solve depression. We need to be very, very clear about that,” said Forger, a 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.”

Strong link between sleep and mood

While the link between sleep and mood is well-known in a casual sense – people might say someone is “cranky” or “fussy” after poor rest – previous studies have linked sleep issues to serious mental health concerns, including elevated 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.”

However, these findings often stem from controlled laboratory settings. Forger and his team aimed to observe how these interactions play out in everyday life. 

Sleep, circadian rhythms, and mood 

The team leveraged data from the Intern Health Study, funded by the National Institutes of Health, which monitors first-year medical interns. P

The participants regularly completed mood surveys while wearing fitness trackers – Fitbits – to measure heart rate, activity, and sleep patterns. 

By applying specialized algorithms to the Fitbit data, the researchers extracted insights into each individual’s circadian rhythms, sleep cycles, and how well these were aligned. 

When combined with daily mood assessments and quarterly depression screening questionnaires (such as the widely used PHQ-9), the team could correlate patterns of sleep alignment with tangible measures of mental health.

They found that when sleep and internal rhythms drifted apart, PHQ-9 scores often rose by an average of 2.5 points – a clinically meaningful change.

Aligning sleep and internal clocks

One of the study’s lead authors, undergraduate researcher Minki Lee, emphasized the importance of timing rather than just sleep duration or start time.

“It’s not just, ‘If you go to bed earlier, you will be happier,’” Lee said. “To some degree, that will be true, but it will be because your sleep schedule is aligning with your internal rhythms.”

The team examined three key patterns: the body’s central circadian clock in the brain, the peripheral circadian clock (such as that in the heart), and the interns’ sleep cycles. 

Typically, the heart’s peripheral clock anticipates more daytime activity than nighttime activity. The researchers discovered that having sleep schedules out of sync with this peripheral clock tended to negatively affect mood.

Interestingly, a misalignment between the central circadian rhythm and sleep was especially problematic for those engaged in shift work. When work demands led to a significant disconnect between sleep timing and the central internal clock, mood issues intensified.

Co-lead author Dae Wook Kim conducted the study as a postdoctoral fellow at U-M and is now an assistant professor at the Korea Advanced Institute of Science and Technology.

“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 Professor Kim.

Context matters for circadian disruptions

These findings challenge earlier assumptions that all circadian disruptions affect mood similarly. 

Understanding how misalignment operates in real-world conditions – like the demands of working interns – could explain why some people cope better with “all-nighters” or jet lag without long-lasting emotional fallout.

The team plans to apply their research methods to other groups, such as students, older adults, and individuals diagnosed with psychiatric disorders, to better understand when and why these disruptions matter.

“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.

Understanding how sleep disruptions impact mood

The good news is that everyday technology – like wearable fitness trackers – can reveal these hidden patterns of circadian misalignment and guide interventions that restore better sync between sleep and internal clocks.

“That’s why this is scalable,” Forger said. “That’s why I think this could help tons of people.”

By understanding when and how sleep disruptions truly impact mood, individuals and clinicians can take steps to preserve emotional well-being – even during the busiest times of the year.

The study is published in the journal npj Digital Medicine.

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