Smartwatch data reveals resting heart rates follow hidden biological rhythms

Your smartwatch has been quietly recording something your doctor never asked about: the slow, hidden rhythm of your resting heart rate. A study of more than 600 college students wearing fitness trackers over two to four years found that seven out of ten of them had resting heart rates that rose and fell in predictable patterns—weekly cycles, monthly cycles, even rhythms that stretched across ten weeks or six months. These weren't random fluctuations. They were genuine biological cycles, as real and measurable as the circadian rhythm that governs your sleep, but operating on a timescale most people never notice.

Resting heart rate—the number of beats per minute when you're calm and sitting still—is a marker of stress, fatigue, and physical fitness. It's also, it turns out, far more complex than anyone realized. Researchers at the University of Notre Dame analyzed years of smartwatch data using a technique called spectral analysis, essentially breaking down each person's heart rate patterns the way a prism splits light into colors. What emerged was striking: most people had a dominant rhythm all their own, and across that rhythm, their resting heart rate could swing by as much as 15 beats per minute from peak to trough.

The cycles sorted themselves into three broad groups. About 13 percent of participants showed weekly patterns—their heart rates dipped and climbed on a seven-day schedule. Thirty-seven percent had monthly cycles. The largest group, half of all participants, had longer rhythms centered somewhere between ten weeks and six months. These weren't tied to a single cause. Some weekly cycles aligned neatly with weekends, when people's schedules shifted. Summer brought lower resting heart rates for many, suggesting seasonal influence. And yes, some women's monthly heart rate cycles tracked with their menstrual periods. But here's where the picture got complicated: one in five people with monthly cycles were men, which meant reproductive hormones couldn't explain the pattern. The body, it seemed, was keeping time by mechanisms science hasn't yet fully mapped.

The discovery challenges a fundamental assumption about human biology—that we operate on a few well-understood rhythms. We know about circadian cycles, the 24-hour pulse that syncs us to day and night. We know about infradian rhythms in other contexts: the female reproductive cycle, migration patterns in birds, hibernation in bears. But a hidden, multi-week or multi-month rhythm in resting heart rate? That was new. And it was universal enough to matter. The fact that people in the same friendship groups tended to show similar heart rate cycles suggested that external cues—light, schedules, social routines—act as a kind of nudge, helping the body lock onto a rhythm. Yet even without those cues, something inside kept the rhythm going.

The implications ripple outward. If resting heart rate follows these slow cycles, then symptoms that seem random—a seizure, a psychiatric episode, heart palpitations—might actually cluster at certain points in a person's rhythm. Doctors could potentially use this knowledge to forecast high-risk periods or time interventions more precisely. The research team is already running clinical trials on digital tools designed to track seizure cycles, part of a larger international effort called My Seizure Gauge. As smartwatches become ubiquitous, they're turning millions of people into unwitting participants in a vast, continuous study of human time. The data is there. The patterns are there. What remains is learning to read them—and to understand what the body is trying to tell us when it speaks in rhythms we can't yet hear.