Summary: Analysis of Fitbit data from more than 2,500 medical interns shows that mood follows a clear daily cycle: it tends to be lowest near 5 a.m. and highest around 5 p.m. The study finds that this circadian pattern is present even without sleep loss, while sleep deprivation independently deepens low moods and increases mood variability across the day.
Using continuous wearable data collected over two years—including heart rate, step counts, sleep metrics, and daily self-reported mood—the research offers real-world evidence of how circadian rhythm and time awake interact to influence emotional state. These results have implications for wellbeing strategies in high-stress, irregular-schedule professions such as medical training.
Key Facts:
- Mood exhibits a robust circadian rhythm, with a trough in the early morning (around 5 a.m.) and a peak in the late afternoon (around 5 p.m.).
- Sleep deprivation independently worsens mood and amplifies daily mood swings, meaning longer time awake leads to lower mood and greater fluctuations.
- The findings are based on continuous wearable monitoring of 2,602 first-year physicians across 168,311 days of data, making this one of the largest real-world studies linking sleep, circadian timing, and mood.
Source: University of Michigan
It’s often darkest before the dawn for many people, and a new study from the University of Michigan and Dartmouth Health has quantified that low point in mood using wearable data.
The study examined Fitbit recordings from more than 2,500 medical interns over a two-year period and matched those objective measures with a daily single-question mood rating. Investigators found a consistent pattern: interns’ self-reported mood tended to be lowest near 5 a.m. and highest near 5 p.m. In addition, being awake for longer periods—indicative of sleep deprivation—further lowered mood and increased the amplitude of mood’s daily cycle.
“Mood naturally cycles, with a lowest point in the early morning and a highest point in the evening, even independent of sleep loss,” said Benjamin Shapiro, lead author and psychiatrist at Dartmouth Health. “Sleep deprivation is a separate factor that further reduces mood.”
“Someone who has been awake all night and is still up at 5 a.m. will likely report a lower mood than someone who simply woke up at 5 a.m.,” Shapiro added. “But even on a regular day, the early morning mood tends to be lower than the evening mood.”
Published in the journal PLOS Digital Health, the study used minute-by-minute wearable heart rate and motion data to estimate circadian phase and time awake. Researchers applied linear mixed models to relate mood ratings to circadian timing and hours awake, demonstrating statistically significant effects of both internal clock signals and accumulated wakefulness on mood.
“We found that mood follows a rhythm tied to the body’s internal clock and that the clock’s influence grows the longer a person remains awake,” said Danny Forger, senior author and professor of mathematics and computational medicine and bioinformatics at the U-M Medical School. “Wearable technology offers a practical approach to studying these relationships outside the laboratory.”
Participants were first-year physicians enrolled in the Intern Health Study who completed a daily mood assessment consisting of a single question: “How was your mood today?” That single-item daily measure was plotted against each participant’s estimated circadian phase and time awake to reveal the consistent pattern of morning lows and evening highs, as well as the worsening effect of prolonged wakefulness.
The authors note several limitations: the analysis generated a generalized mood model for medical interns and does not capture every individual’s unique pattern, social factors, work schedules, and temperament can alter mood dynamics. Few participants remained awake more than 18 hours in a single day, and the study did not use multi-item validated scales like the Depression Anxiety Stress Scale or other clinical screening tools.
Still, the study highlights the promise of noninvasive wearables for monitoring circadian rhythms and mood trends. “Instead of invasive measures like blood draws or continuous temperature probes, consumer-grade wearables can supply valuable data,” Shapiro said. “This opens opportunities for clinicians to incorporate circadian metrics into everyday mental health practice and to explore chronotherapy interventions.”
About this mood and sleep research news
Author: Morgan Sherburne
Source: University of Michigan
Contact: Morgan Sherburne – University of Michigan
Image: Image credited to Neuroscience News
Original Research: Open access. “Unraveling the interplay of circadian rhythm and sleep deprivation on mood: A Real-World Study on first-year physicians” by Benjamin Shapiro et al., PLOS Digital Health
Abstract
Unraveling the interplay of circadian rhythm and sleep deprivation on mood: A Real-World Study on first-year physicians
Interactions among circadian rhythms, time awake, and mood are not well characterized in real-world settings, yet they are especially relevant for people in demanding jobs with irregular hours. Wearable devices now enable large-scale, noninvasive study of these processes outside laboratory conditions. This study analyzed continuous heart rate, step counts, sleep measures, and daily mood ratings from 2,602 medical interns across 168,311 Fitbit-recorded days. Circadian phase and time awake were inferred from minute-by-minute wearable heart rate and motion data. Linear mixed models revealed that mood is significantly modulated by circadian timing (p<0.001). Increasing time awake both worsens mood (p<0.001) and nonlinearly amplifies mood’s circadian rhythm. These findings demonstrate how circadian rhythms and sleep deprivation jointly shape mood and show the feasibility of using consumer wearables to study these factors in real-life settings, suggesting potential for chronotherapy approaches in psychiatric care.