Data from more than 600 people shows resting heart rates have their own secret rhythm

You’re likely familiar with the idea of a circadian rhythm – your body’s 24-hour cycle that governs sleep and wakefulness.

It turns out that resting heart rate also follows mysterious, long rhythms that run independently of the outside world. Resting heart rate is measured by how many beats per minute your heart makes while you’re calm and inactive. This measure is linked to stress, fatigue and fitness.

Our new study, published in EBioMedicine, looked at years of smartwatch data from hundreds of young adults, and found that for 70% of the participants, their resting heart rate followed a slow rhythm.

These rhythms had distinct weekly, monthly, or even ten-week cycles, where the number of heart beats per minute would rise and fall in a defined pattern within that period.

Identifying these cycles expands what we know about human biological time and could be used to improve individuals’ health.

Our many internal clocks

Human biological time refers to our bodies’ systems for measuring time, separate from a clock or calendar. Our different inner timing becomes more obvious when we move across time zones or experience jetlag.

Biological time is set by internal “clocks” in the brain and other organs, which govern bodily functions – from hunger to hormone levels. Similar clocks are found across most living organisms and interact with the outside environment such as sunlight, moonlight, and tidal or seasonal cycles.

The circadian (24-hour) rhythm is the most well known, but longer cycles – called infradian or “slower than daily” – are also observed in nature. The female reproductive cycle is an example of an infradian rhythm, as are migration or hibernation patterns in some animals.

Previous research has considered how infradian rhythms impact disease symptoms, including epileptic seizures, heart attacks and psychiatric episodes. Tracking these cycles could help forecast high-risk periods or be used to target therapies. As part of the international My Seizure Gauge consortium, our team developed digital health tools to track seizure cycles, with clinical trials currently underway.

Now, people’s increasing use of smartwatches is turbocharging research into human rhythms and how we measure them at scale.

What did we discover?

Detecting infradian rhythms requires years of data, because everyone’s cycle is unique, and the pattern takes a long time to repeat. This makes them more difficult to measure than circadian rhythms, which are similar for everyone and repeat every day.

Heart rate cycles, common in males and females, represent a new type of infradian rhythm that cannot yet be explained by science, but nevertheless affects our wellbeing.

For this study, we used data collected by the NetHealth project at the University of Notre Dame, United States. With the help of smartwatches, it tracked about 600 college students for between two and four years.

Our study analysed people’s daily resting heart rate over time using a technique called spectral analysis. You can think of this like splitting light into a rainbow, where each colour represents a different cycle (red cycles are slower; violet cycles are faster). The aim is to identify each person’s dominant “colour”.

Most people in the study (369 out of 525 participants) did have a dominant resting heart rate rhythm, ranging from weekly to yearly cycles. Across their cycle, resting heart rate changed by as much as 15 beats per minute from the peak to the trough.

Although everyone had their own rhythm, three groups emerged with cycles centred in either a weekly (13% of people), monthly (37%), or longer “multi-month” (50%) range.

Synchronising with our environment

Some findings were consistent with what we already know about heart rate trends. Yearly cycles tended to align to seasons (lower resting heart rates in summer) and weekly cycles tended to align to weekends.

Females’ monthly heart-rate cycles were often aligned to their menstrual periods. However, 20% of the monthly heart rate cycle group were male, suggesting even monthly cycles can’t be simply explained by reproductive hormones.

These findings matter for women’s health, highlighting that monthly symptoms don’t necessarily relate directly to menstrual cycles. To properly identify and treat the cause of cyclic symptoms (which could include seizures, psychiatric episodes or heart palpitations) it’s important to consider the effect of infradian rhythms beyond the reproductive system.

More broadly, the evidence points to calendar cycles acting as a “nudge” rather than the cause of heart rate rhythms. Because biological rhythms are designed to synchronise with our environment, heart rate cycles probably lock onto certain cues, such as light levels, or weekly schedules, to help the body keep time.

Likewise, individuals from the same friendship groups also tended to show more similar heart rate cycles. But even without these environmental or social cues, internal mechanisms continue to synchronise our body to a slow rhythm.

The unexpected diversity of cycles, including multi-month rhythms centred at ten weeks and six months, challenges the idea of human biology having a fixed baseline.

The study opens crucial questions about what else might be changing alongside our slowly oscillating heart rates. At our lab, we’re investigating how people’s mood, stress and disease risk are affected by infradian rhythms. If you’re a smartwatch user, you can even upload your own data to find out if your heart has a hidden rhythm.