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........

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