HRV (Heart Rate Variability)

What HRV is

Heart rate variability is the variation in time between consecutive heartbeats, measured in milliseconds. A heart beating at 60 bpm doesn’t beat precisely once per second — the gaps between beats vary slightly. That variation is HRV.

Counterintuitively: higher HRV is better. A higher HRV means your heart is more flexible, your nervous system is in parasympathetic (rest-and-digest) mode, and your body is recovered. Low HRV means your system is under stress — from training, illness, poor sleep, alcohol, or psychological pressure.

Why workout apps use it

HRV is the only non-invasive proxy for recovery status that correlates with how your body will actually perform.

Resting heart rate tells you something. Sleep duration tells you something. HRV integrates both signals plus additional nervous system state into a single number. Apps like Whoop measure HRV during sleep (when it’s most stable) to produce a daily recovery score.

What the numbers mean

HRV is highly individual — there is no “good” HRV number that applies across people. A 22-year-old athlete might average 95ms; a 45-year-old office worker might average 38ms. Both can be perfectly healthy for their baseline.

What matters is your own trend. A reading significantly below your personal baseline (flagged by Whoop, Garmin, or Oura as “low recovery”) indicates the need for easier training. A reading at or above baseline indicates readiness for hard effort.

Which apps use HRV well

Whoop: measures HRV with 3 samples during sleep, averages them, compares against your rolling 30-day baseline, and produces a recovery score (0–100%). The most actionable HRV implementation for training decisions.

Garmin Connect: includes HRV status in mid-to-high tier devices (Forerunner 265+ and Fenix series). Less granular than Whoop but free after hardware purchase.

Oura Ring: hospital-grade HRV tracking at ring form factor; favoured for sleep-primary users.

Apple Watch: captures HRV but doesn’t produce a recovery score — raw data only.

Related concepts

• Progressive overload — understanding HRV lets you push progressive overload when recovered and back off when not
• Active recovery — low HRV days are active recovery days, not rest days
• Resting heart rate — a companion recovery metric; both together are more predictive than either alone
• VO2 max — a separate fitness ceiling metric, not a recovery metric