Heart Rate Variability (HRV) Explained Simply

Your watch gives you an HRV number, sometimes with a colored status. You're not sure what to make of it, or whether you should change your workout because of it.

That number is useful if you know what it measures and what it doesn't tell you. Here's what matters, and what you can do with it starting tomorrow.

HRV at a glance: a heart that isn't a metronome

At 60 beats per minute, your heart doesn't beat exactly once every second. The gap between two beats changes by a few milliseconds. That variation is HRV.

At rest, your autonomic nervous system slows the heart, which would otherwise beat at around 100 beats per minute. It has two branches:

• the parasympathetic slows the heart and raises HRV;
• the sympathetic, the alert system, speeds it up and lowers HRV.

HRV reflects the balance between the two. It is not a fitness score.

What your watch actually measures

On the wrist there are no electrodes. An optical sensor reads your pulse through the skin. It measures the time between two pulses, not between two electrical heartbeats. That's close, but not identical.

So an absolute HRV value from your wrist is approximate. Compare your nights with each other, using the same watch on the same wrist, and don't switch devices midway.

Two acronyms may show up in your app: RMSSD and SDNN. They are two ways of summarizing the gaps between beats in a single number. Check your brand's help page to see which one is displayed, and don't mix them up.

Why comparing yourself to a friend makes no sense

A large real-life analysis links HRV to age, sex, BMI, physical activity level and daily stress. Two people don't start from the same point. Your only real reference is you.

Reading your trend: the 7-day average

On Garmin, the watch records your HRV every night for three weeks to set your baseline range. After that, "Balanced" means your 7-day average is within that range, and "Low" means it's well below it. A single night's reading can swing for any number of reasons, and a weekly average smooths out that noise. Other brands (Apple, Coros, Polar, Suunto) have their own calibration and display rules. Check them in their help pages.

The way the Garmin status works comes from Garmin's documentation. The advice to ease off a workout comes from coaching practice, not from a clinical trial.

High HRV doesn't mean you're in good shape

In elite endurance athletes, both rises and drops in HRV have been linked to poor training adaptation, and better fitness has sometimes coincided with lower HRV. The authors (Plews et al., 2013) advise averaging measurements and tracking each athlete's own individual pattern.

Training with HRV: what the studies say

These reviews focus on endurance athletes. A 2021 meta-analysis compared HRV-guided training with a predefined plan:

• vagal-related HRV indices improved more with HRV-guided training;
• for resting heart rate, there was no significant difference between the two groups;
• for endurance performance, an advantage was not shown.

A 2020 meta-analysis on VO2max adds nuance: both groups improved their VO2max, with a larger effect in the HRV-guided group. So the results contradict each other. Remember that HRV can help you track fatigue, but it doesn't replace a good training plan.

What you can do before your next workout

• ✓Wear your watch every night, on the same wrist, with the same device.
• ✓Give your watch time to calibrate: on Garmin, three weeks. For another brand, read its help page.
• ✓Look at the 7-day average, not the number from this morning.
• ✓Write one sentence about how you feel before you head out.

KILYO reads your watch workouts and analyzes pace, heart rate, volume and consistency, then adjusts your training plan. It doesn't read your HRV, but it helps you see whether your fatigue is confirmed: a pace that drops, or a higher heart rate at the same pace, from one workout to the next.

Written with the help of AI.