What is HRV: meaning, normal values and how to read it

HRV (heart rate variability) doesn't measure how fast the heart beats, but how irregularly: and that millisecond-level irregularity between beats is a direct signal of autonomic nervous system state. Comparing your value to others is almost always useless: what matters is your personal trend over time, not the absolute number.

What is HRV: the simple definition

The heart doesn't beat like a perfect metronome. Even if your heart rate is 60 BPM, individual beats don't happen exactly every second: one arrives after 980 ms, the next after 1,020 ms, the one after that after 990 ms. This variation in intervals between beats is called heart rate variability (HRV). You can calculate RMSSD and standard deviation on your own intervals with the FitMesh Labs HRV calculator, entirely in your browser.

The wider this variation (meaning the more 'irregular' the beats are in interval), the higher your HRV. This seems counterintuitive because cardiac irregularity is usually associated with problems. But here we're talking about physiological variations in the order of milliseconds, not pathological arrhythmias. Higher HRV is often associated with a well-regulated autonomic nervous system and a good recovery state, but it isn't direct, isolated proof: the value also depends on the metric used, measurement duration, sensor, device algorithm, context (e.g. illness, alcohol, sleep), and possible measurement artifacts (movement, missed beats). That's why your personal trend, measured under consistent conditions over time, matters more than any single number. The type of metric reported also varies by device: RMSSD measures short-term variability between consecutive beats, SDNN is the standard deviation of the entire series of normal intervals after removing artifacts, and many wearables instead show a proprietary score that combines multiple signals. These values aren't directly comparable to each other.

In formal terms: RMSSD = the square root of the mean of the squared differences between consecutive RR intervals. You can see this formula applied step by step to your own data, with a full worked example, in the FitMesh Labs HRV Calculator.

Why HRV reflects the autonomic nervous system

The autonomic nervous system has two branches: the sympathetic (activation, stress, 'fight or flight' response) and the parasympathetic (recovery, rest, 'rest and digest' response). The heart is influenced by both constantly, and that variation in beat timing is the result of this dual influence balancing moment by moment.

When you're under chronic stress, haven't slept well, had an intense workout last night, or are starting to get the flu, the sympathetic system takes over: the heart beats more regularly, variability drops. When you're rested, well-recovered, and the parasympathetic system is dominant, variability rises.

How wearables measure HRV

Consumer wearables (Galaxy Watch, Garmin, Oura, Polar) primarily use two methods to measure HRV:

• •PPG (photoplethysmography): a green (or red/infrared) LED illuminates the wrist and a sensor measures light absorption variations caused by blood flow. This is the method used by the vast majority of smartwatches. Less precise than ECG, but sufficient for trend tracking.
• •ECG: available on some premium watches (Galaxy Watch Ultra, Apple Watch Series with ECG function). Directly measures the heart's electrical activity. More accurate for short sessions, but the practical difference for daily tracking is often marginal.

The most reliable measurement a wearable can make is during sleep, when you're still and there are no movements distorting the signal. Garmin calls this metric 'overnight HRV' or 'nightly average RMSSD'. Oura does the same. Galaxy Watch (via Samsung Health) records HRV during sleep and exposes values in the app.

What to do (and not do) with HRV in daily training

The most widespread use of HRV among amateur athletes is as a recovery indicator: if HRV is significantly below personal baseline, it might not be the best day for an intense workout. This is a useful simplification, but with some important conditions.

• •It makes sense to look at it as a weekly trend, not as an absolute daily figure. A single night of low HRV can have a hundred causes (heavy dinner, alcohol, atypical sleep position, hot night). A full week below your baseline is more informative.
• •It's not a disease indicator: low HRV doesn't mean you're about to get sick or have a heart problem. It's a signal of autonomic nervous system state, not a diagnosis.
• •Cross-app comparison isn't direct: Garmin, Samsung, Oura and Polar use different algorithms and measurement intervals. A value of 55 on Garmin isn't directly comparable to a 55 on Oura, even if both measure 'HRV'.
• •It doesn't replace common sense: if you slept 5 hours but your HRV is high that morning, the sleep debt you need to recover probably matters more than the HRV number.

HRV and general health: the frequently asked questions

Beyond sports, HRV is used in clinical contexts as a marker of chronic stress, cardiovascular wellness, and treatment response in some conditions. This doesn't mean the number on your smartwatch carries the same clinical weight as a medical measurement: precision is different, measurement protocols are different, and a doctor uses HRV in very specific contexts with calibrated instruments.

The value of the wearable is in personal longitudinal tracking: seeing your own patterns over time, noticing variations correlated to life events (work stress, routine changes, travel), and using this information to make more informed decisions about lifestyle. Not for self-diagnosis.

In summary

• •HRV measures variation in beat intervals (milliseconds), not average heart rate: higher variability is often associated with a well-regulated autonomic nervous system, but it also depends on the metric, duration, sensor, algorithm, context, and artifacts, it isn't standalone proof.
• •The absolute value varies enormously between people: comparing it to others isn't useful. What matters is your personal trend over 2-4 weeks, measured under similar conditions.
• •The most reliable measurement is overnight (during sleep), when movement doesn't disturb the PPG sensor.
• •Garmin, Oura, Samsung and Polar values are not directly comparable: they use different algorithms. Pick one device as your reference and don't mix numbers.
• •To improve HRV over time: regular sleep, moderate aerobic activity, alcohol reduction. There are no quick hacks: visible changes take weeks.