Glossary

What is HRV?

Short answer

HRV (heart rate variability) is the variation in time between consecutive heartbeats, measured in milliseconds. Higher-than-usual HRV generally reflects a more recovered nervous system, but the absolute number is highly individual — it's only meaningful compared with your own baseline.

Also called: Heart rate variability · SDNN · rMSSD

Last reviewed

What HRV actually measures

Your heart doesn’t beat like a metronome. Even at a steady 60 beats per minute, the gap between one beat and the next might be 0.95 seconds, then 1.04, then 0.99. HRV quantifies that variation.

The variation comes from the two branches of the autonomic nervous system:

  • one speeds the heart up (sympathetic)
  • one slows it down (parasympathetic)

When you’re rested, the two constantly adjust heart rate, which produces more beat-to-beat variation. Under stress, illness or accumulated fatigue, that adjustment narrows, and HRV tends to fall.

The two numbers you’ll see

  • SDNN is the standard deviation of the intervals between normal beats. It’s what Apple Watch reports in Apple Health.
  • rMSSD is the root mean square of successive differences. Many other wearables and HRV apps report it.

They’re related but not interchangeable. A number from one device or metric shouldn’t be compared with a number from another.

Why your HRV isn’t comparable to anyone else’s

Age, genetics, fitness, breathing, posture and the measurement method all shape the absolute value. Forty milliseconds can be high for one person and low for another. The useful comparison is today versus your own rolling baseline, built over weeks, on the same device, measured the same way. The overnight reading is the most stable one.

How to use it

Read HRV with context, not as a green or red light:

  • sleep
  • recent training load
  • illness
  • alcohol
  • how you actually feel

A multi-day dip with no obvious cause is worth acting on. A single low morning after a hard session usually isn’t. The HRV, sleep and training-readiness guide walks through that decision. The Apple Watch HRV guide covers how accurate wrist readings actually are.

Go deeper

Common questions

What is a normal HRV?

There isn't one normal number. HRV varies widely with age, genetics, fitness, and even the device and metric used (Apple Watch reports SDNN; many other wearables report rMSSD). What's normal is whatever your own baseline is over several weeks.

Is a low HRV bad?

A single low reading usually isn't. A reading well below your own baseline, lasting several days, can reflect accumulated fatigue, poor sleep, stress, or illness — a prompt to look at the rest of your week, not a diagnosis.

Should I skip training when my HRV is low?

Not automatically. Check for an obvious cause (a bad night, a hard session, alcohol, illness) and how you feel. A persistent dip with no clear cause is a better reason to reduce training than a single morning's number.

Sources

  1. Training adaptation and heart rate variability in elite endurance athletes: opening the door to effective monitoring · Sports Medicine / PubMed
  2. Heart Rate Variability Applications in Strength and Conditioning: A Narrative Review · Journal of Functional Morphology and Kinesiology / PubMed
  3. The Validity of Apple Watch Series 9 and Ultra 2 for Serial Measurements of Heart Rate Variability and Resting Heart Rate · Sensors / PubMed Central

Educational content only; not medical advice or individualized professional guidance.

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