What is heart-rate variability?

Heart-rate variability, HRV, is the variation in time between consecutive heartbeats, measured in milliseconds. A resting heart is not a metronome: the gap between beats lengthens and shortens with breathing and with the balance of the nervous system, and a larger variation generally reflects a body that is recovered and ready to be loaded. A smaller variation than your own usual is one of the earlier signs of fatigue, illness, or a late night.

Two things follow. It is a comparison against your baseline, not a league table; and it moves for reasons that have nothing to do with training, which is why it is one input among several rather than a verdict. Measured overnight or at rest in the morning, a typical adult SDNN sits somewhere near 50 milliseconds, and the day-to-day swing around that is larger than most people expect — which is why one low reading means less than a week of them.

SDNN or RMSSD: which HRV number is it?

There are several ways to summarise the variation, and the two you will meet are not interchangeable. SDNN is the standard deviation of all the beat-to-beat intervals in a window; RMSSD is the root mean square of the differences between successive intervals, which weights the fast, breathing-driven changes. Apple Health reports SDNN. Android’s Health Connect reports RMSSD, which runs lower — a healthy median around 42 ms against roughly 50 ms for SDNN. Comparing one against a baseline built from the other tells you nothing.

Liftor keeps the two apart. Each HRV reading is compared only against a baseline of the same metric from the same source, and until you have seven days of your own readings it uses a population median for that metric so the maths never divides by nothing.

How does Liftor use HRV?

As fifteen percent of the readiness score. When you allow it, the app reads HRV, resting heart rate and sleep from Apple Health each morning, compares each against your rolling baseline, and folds the result into one number alongside the check-in you type. HRV is not the biggest term — sleep and your own check-in each carry twenty-five — and it cannot push the score below the floor on its own. What it does is catch the morning you would have called fine.

What is a good HRV?

Yours, on a good week. The published short-term norms for healthy adults, collected by Shaffer and Ginsberg in 2017, put SDNN at a mean of 50 ms with a range of 32–93 and RMSSD at 42 ms with a range of 19–75 — which is where Liftor’s starting medians come from — and the 1996 Task Force put a 24-hour SDNN above 100 ms in the healthy band. The ranges are wide because the number falls with age, rises with aerobic fitness, and differs between devices, so two people with the same reading are not equally recovered and one person on two mornings might be.

The comparison that carries information is you against your own rolling baseline. A reading ten percent under your usual, for three mornings, is worth acting on; a reading under a stranger’s is not. Liftor builds the baseline from your own seven days as soon as it has them, and reads only the direction and the size of the gap.

Why does HRV drop after a hard session, a late night or a drink?

Because all three shift the nervous system toward its alarm setting. Variability between beats is mostly the parasympathetic branch lengthening and shortening the gaps as you breathe; hard training, short sleep, alcohol, illness and stress each pull the balance the other way, the heart beats more evenly, and the number falls — usually overnight, and usually before you feel it. That is what makes it useful, and also what makes a single reading unreliable: a big meal or a warm room will do it too.

Liftor treats the causes it can see separately rather than reading them twice. Alcohol the night before subtracts seven percent from the readiness score on its own; the last 48 hours of training carry fifteen; HRV carries fifteen more. So a drink after a heavy leg day shows up as three small terms with three names, and the morning’s note says which one moved.

How should you measure HRV so the trend means something?

Same device, same conditions, every day, and then ignore any single reading. An Apple Watch takes HRV in the background, mostly while you sleep or during the Breathe app, and reports SDNN; a chest strap or a ring measures differently and reports differently, and neither is comparable to the other. Pick one, wear it to bed, and let a week accumulate. Morning readings taken sitting up after coffee are a different measurement from overnight ones, and mixing the two makes a baseline out of noise.

The app reads whatever Apple Health holds and compares each reading only against a baseline of the same metric from the same source. Until seven of your own days exist it uses the population median for that metric, so the first week’s score is honest about being an estimate rather than a verdict.

Sources

Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Frontiers in Public Health, 2017; 5: 258
Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation, and clinical use. Circulation, 1996; 93(5): 1043–1065
Apple Developer: HKQuantityTypeIdentifier.heartRateVariabilitySDNN — HealthKit reports HRV as SDNN
Liftor’s readiness model: metric-matched baselines, the 50 ms SDNN and 42 ms RMSSD medians, the seven-day baseline rule (services/readiness.ts)