Your heart rate is more uneven than you think. This is what it says about your health - BBC

Micro-fluctuations in the time between heartbeats are proving a helpful indicator of mental health, stress levels and exercise capacity. They could even provide insight into how well you are ageing.

Artem Kirillov is not, by nature, the kind of person to take it easy. "I prefer to push myself further, even if I feel a bit off," says Kirillov, who is 40, lives in London and works in health tech. For a long time, even as a recreational exerciser, he discounted the need for rest days and pushed through fatigue at the gym, believing that more time spent training automatically meant better results.

Until, that is, he began paying attention to heart rate variability, one of the many health data points tracked by his smartwatch. A more complex metric than heart rate – the number of times the heart beats per minute – heart rate variability reflects how the time between heartbeats fluctuates. A growing body of research suggests it's an indicator of cardiovascular health, stress levels, exercise capacity and more, allowing dedicated trackers to make more informed decisions about their fitness regimes and lifestyles.

Now, if Kirillov is on the fence about whether it's better to take a day off or grind it out in the gym, he consults his heart rate variability score. Since adopting that habit, "I feel like I'm in better balance with myself", he says. He's such a convert, he even launched an app dedicated to tracking stress using heart rate variability data.

As wearables become ever-more ubiquitous and research on heart variability accumulates, more people are joining Kirillov in keeping this score, says Deepak Bhatt, director of the Mount Sinai Fuster Heart Hospital in New York City, US.

But do you actually need to track heart rate variability, and what can you learn if you do? Here's what to know.

What is heart rate variability?

"You want a heart to beat more or less regularly," Bhatt says. When the heart beats extremely irregularly, it's classified as an arrhythmia, which in serious cases can result in complications such as stroke or heart failure.

But even a healthy heart has some variation in the time between its beats, Bhatt says. These variations are tiny, measured on the order of milliseconds (one millisecond is a thousandth of a second). And when looking at changes on this scale, "a higher variability, in general, is considered better" than a lower one, Bhatt says.

There's no single ideal heart rate variability score, as it varies by age, fitness level, sex, tracking device and calculation method. But one wearable brand says the average score for its users, who tend to be active and health-conscious, is 65 milliseconds for men and 62 milliseconds for women. And there's huge variation by age group: the average score for 25-year-olds is 78, compared to 44 for 55-year-olds.

While the maths behind these numbers is complex, you can think of them as approximations of the average fluctuation in the time intervals between heartbeats in milliseconds.

Shooting for a high heart rate variability may seem counterintuitive, since a low resting heart rate suggests someone has good cardiovascular fitness. High heart rate variability, though, is a way to measure how well your nervous system is cycling between its "fight-or-flight" stress response and its "rest-and-digest" relaxation response.

Here's how it works. If you need to outrun a predator – or just go out for a jog – your nervous system triggers a range of physiologic responses that give you energy and acuity. Among other effects, your heart rate rises. When it does, your heart rate variability drops because the heart has to keep beating at a fast and steady pace to sustain you.

When you're back at rest, the nervous system should calm everything back down. In this relaxed state, your heart rate naturally beats at a more variable pace – for example, speeding up a little when you inhale, then slowing down when you exhale.

A high average heart rate variability "shows that your system can, when it needs to, quickly change heart rate and blood pressure to match the environment or to match the circumstances", says Dennis Larsson, a postdoctoral research fellow at Kiel University in Germany who has studied heart rate variability. This suggests it can spring into action when something is stressful but relax again when something doesn't have to be stressful.