Most people know that the body runs on a daily rhythm. Hunger, alertness and sleepiness all tend to rise and fall over roughly 24 hours. But new research suggests that the human body may also follow much slower biological cycles that unfold across weeks or even months.

A study published in eBioMedicine analysed more than 900 years of smartwatch data from 623 healthy university students. Researchers found that the resting heart rates of around 70% of participants followed recurring long-term patterns, including weekly, monthly, quarterly and yearly cycles.

Heart Rate Does Not Stay Random

The research focused on so-called infradian rhythms. Unlike circadian rhythms, which repeat about once every 24 hours, infradian rhythms last longer than a day and can span several days, weeks or months.

These longer cycles are already known to influence processes such as reproduction and seasonal changes in mood. They have also been linked to seizure risk in epilepsy, where researchers are testing whether identifying vulnerable phases could eventually help time treatment more precisely.

In the new study, many participants showed distinctive heart-rate cycles lasting a week, a month, 10 weeks, six months or even longer. Importantly, the patterns could not be fully explained by season, exercise or environmental conditions.

More Than 900 Years Of Wearable Data

The team, led by Associate Professor Pip Karoly at the University of Melbourne, examined data from students enrolled in the NetHealth Project in the United States.

By combining years of continuous smartwatch measurements, the researchers were able to detect repeating patterns that would have been almost impossible to see in a short laboratory experiment.

Karoly explained that the results suggest many people have individual long-term heart-rate rhythms that operate alongside the better-known daily body clock. According to her, understanding these slower cycles could reveal periods when a person is more vulnerable to stress or changes in neurological and psychiatric symptoms.

Men And Women Showed Different Patterns

The timing of these cycles appeared to differ between men and women. Men were more likely to show weekly and multi-month rhythms, while women more often displayed monthly patterns. However, the researchers stressed that both men and women showed a wide variety of cycles and that these rhythms were not simply driven by the day of the week, season or lunar phase. An especially intriguing result involved social relationships. Housemates, siblings and friends tended to show more similar heart-rate rhythms than unrelated people. That finding does not prove that close relationships directly synchronise the heart. However, it raises the possibility that shared routines, environments or other social influences may shape longer biological rhythms.

Scientists Still Do Not Know What Drives Them

The biological mechanism behind these cycles remains uncertain.

Researchers suspect that hormonal, environmental and genetic factors may all contribute. One possibility is that longer rhythms emerge through interactions between multiple circadian clocks located throughout the body, including the brain, heart, skin and gut.

Another possibility is the existence of a distinct slower biological timing system that operates independently of the 24-hour clock.

The current study cannot determine which explanation is correct. It also does not show that particular heart-rate cycles cause disease or predict health problems on their own.

Why Hidden Rhythms Could Matter

The research team is now developing tools to track infradian rhythms alongside neurological and psychiatric patterns and is also investigating the genetics behind these multi-day cycles. Members of the public can already participate through a heart-rate tracking app created by the researchers.

For ordinary smartwatch users, the findings offer a different way to think about heart-rate variation. A change from one day to another may not always reflect stress, poor fitness or illness; it could sometimes be part of a longer personal rhythm.

The study does not yet provide a clinical test or treatment. Its importance lies in showing that human physiology may be organised across much longer timescales than previously appreciated. If future research confirms how these rhythms work, they could eventually help scientists understand when the body is more resilient, when it is more vulnerable and how health changes unfold over time.