Secret Rhythm of Resting Heart Rates Revealed by Data

How Smartwatches Reveal Infradian Heart Rate Rhythms

Human biological time relies on internal clocks in the brain and organs to govern everything from hunger to hormone levels. While the 24-hour circadian cycle is well documented, longer infradian rhythms—meaning slower than daily—are less understood because they require years of continuous tracking to detect. According to the EBioMedicine study, researchers analyzed daily resting heart rate data collected by the NetHealth project at the University of Notre Dame. That project tracked about 600 college students using smartwatches for periods between two and four years.

Using a spectral analysis technique comparable to splitting light into a rainbow of different slow and fast cycles, the research team discovered that 369 out of 525 participants exhibited a dominant resting heart rate rhythm. Across these cycles, an individual’s resting heart rate changed by as much as 15 beats per minute from peak to trough. Participants fell into three primary cadence groups: weekly cycles affecting 13% of people, monthly cycles accounting for 37%, and multi-month ranges spanning up to ten weeks or longer for 50% of the cohort.

Comparing Wearable Datasets: Individual Baselines Versus Population Averages

The Notre Dame findings align with broader population insights gathered from commercial wearable devices. A separate analysis of Fitbit data from over 92,000 users published in PLOS ONE found that normal resting heart rates vary wildly across the general population, ranging between 40 and 109 beats per minute, according to researcher Dr. Mesias Quer and his team. While population averages hover around 65 beats per minute and are influenced by age, sex, body mass index, and sleep quality, both studies emphasize that an individual’s baseline remains remarkably stable over time while still accommodating these newly identified slow biological oscillations.

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Environmental Cues and Internal Timing Mechanisms

While environmental cues act as calendar “nudges” to help the body keep time, they do not entirely dictate heart rate rhythms. Yearly cycles observed in the EBioMedicine study tended to align with seasonal shifts—yielding lower resting heart rates in summer—while weekly cycles often synchronized with weekends. Similarly, female monthly heart-rate cycles frequently aligned with menstrual periods. However, 20% of the monthly heart rate cycle group were male, demonstrating that monthly physiological oscillations cannot be explained solely by reproductive hormones.

The evidence suggests that calendar events merely anchor internal rhythms rather than create them. Individuals within the same friendship groups displayed more similar heart rate cycles, pointing to shared social cues. Yet, even without explicit environmental triggers, internal biological mechanisms continue to synchronize the body to slow, multi-week oscillations that challenge the traditional assumption of a fixed human physiological baseline.

Frequently Asked Questions

What is a resting heart rate?

Resting heart rate measures how many times your heart beats per minute while you are calm, inactive, and resting. It serves as a vital sign indicating cardiovascular fitness, stress levels, and overall health.

What is an infradian rhythm?

An infradian rhythm is a biological cycle that lasts longer than 24 hours. Examples include seasonal migration patterns in animals, the human female reproductive cycle, and the newly discovered multi-week resting heart rate cycles.

How do smartwatches track heart rate rhythms?

Smartwatches use photoplethysmography sensors to detect blood flow changes in the wrist continuously, allowing researchers to apply spectral analysis to years of accumulated daily data.

Can men have monthly heart rate cycles?

Yes.