In plain English
The small, constantly changing gaps between heartbeats. Measured overnight and compared with your own baseline, it's one of the most useful recovery signals a wearable gives you.
What it is
Your heart doesn’t beat like a metronome. The time between one beat and the next shifts by a few milliseconds all the time, as your nervous system speeds the heart up and slows it down. Heart rate variability (HRV) is a measure of that beat-to-beat variation, usually reported in milliseconds.
A large part of that short-term variation comes from the parasympathetic (“rest and digest”) branch of the nervous system acting on the heart through the vagus nerve. That’s why HRV gets used as a window into how stressed or recovered your body is.
Why it matters for training decisions
HRV is most useful as a trend against your own baseline. Published norms vary widely with age, sex, fitness and how the measurement was taken, so comparing your number with a friend’s, or with a population chart, tells you very little.
A run of nights below your usual range during a hard block is worth noticing. It can reflect training load, but also poor sleep, alcohol, illness, travel or life stress. In a small 2007 study, runners who chose hard or easy days from their morning HRV improved at least as much as a group on a fixed plan. That’s encouraging, but it’s one small study, not proof that HRV-guided training works for everyone.
HRV can also mislead. In elite endurance athletes, both rises and falls in HRV have been linked to poor adaptation, which is why researchers recommend rolling averages over single readings.
How wearables measure it, and the limits
- Apple Watch records HRV in Apple Health as SDNN, from short readings taken when you’re still.
- Garmin measures HRV while you sleep and compares your 7-day average with a personal baseline it builds over about three weeks.
- WHOOP and Oura report a sleep-based RMSSD value.
Because devices use different metrics, time windows and algorithms, their numbers can’t be compared directly. A Garmin HRV of 45 and an Apple Watch HRV of 45 aren’t the same thing. Compare a device only with itself.
How P247 uses it
HRV is one of the inputs to P247’s daily recovery score, where it’s rated against a fixed reference value rather than your own baseline. The comparison with your own baseline is a separate HRV status label. It compares each night’s value with your recent normal, using only earlier values from the same measurement source, so a sleep-only reading isn’t judged against a whole-day average.
Read more on the P247 blog
Sources
- Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Frontiers in Public Health, 2017.
- Plews DJ, Laursen PB, Stanley J, Kilding AE, Buchheit M. Training adaptation and heart rate variability in elite endurance athletes: opening the door to effective monitoring. Sports Medicine, 2013.
- Kiviniemi AM, Hautala AJ, Kinnunen H, Tulppo MP. Endurance training guided individually by daily heart rate variability measurements. European Journal of Applied Physiology, 2007.
- Apple Developer Documentation: heartRateVariabilitySDNN (HealthKit).
- Garmin Venu 3 Owner's Manual: Heart Rate Variability Status.
- WHOOP: Heart rate variability: how to read and improve your HRV.
- Liang T, et al. Deriving accurate nocturnal heart rate, rMSSD and frequency HRV from the Oura Ring. Sensors, 2024.
This glossary is general information to help you understand your training data. It isn't medical advice, diagnosis or treatment. For injury, illness, medication, blood test results or any health concern, speak to a qualified health professional.
P247 turns your Apple Health data into a daily brief and a weekly report that tells you what to change next week. Launching on the App Store at the end of November 2026. Your first weekly report is free.
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