HRV: what is heart rate variability?

Heart rate variability (HRV) measures something counterintuitive: your heart doesn't beat like a metronome, and that's a good thing. The interval between two beats varies by a few milliseconds — 850 ms, then 910, then 870. HRV quantifies those variations. As a general rule, the bigger they are, the better.

Why? Because these micro-adjustments directly reflect your autonomic nervous system. The parasympathetic system — the one for rest, digestion and repair — constantly brakes and releases your heart, which creates variability. When your body is under pressure (training you haven't absorbed, a short night, stress, alcohol, infection), the sympathetic system — the alarm system — takes over: your heart beats more “evenly,” and HRV drops. That's why it sits at the core of every readiness score.

How to read your HRV: three rules

• Compare yourself with yourself, never with others. A 2010 review (44 studies, 21,438 healthy adults) puts RMSSD, the main index of parasympathetic activity, at around 42 ms on average, with study averages ranging from 19 to 75 ms and huge individual differences depending on age, genetics and training. Your absolute value says almost nothing; how it changes relative to your average over the past few weeks says almost everything.
• Only measurements taken under the same conditions count. The benchmark is overnight or on waking, at rest. A reading taken standing up after a coffee isn't comparable, and neither is an afternoon reading compared with an overnight one.
• Think in 7-day averages. HRV naturally varies from day to day; an isolated drop is noise, a week of decline is a signal. Researchers who monitor Olympic champions also smooth HRV over several days.

What causes low HRV?

The four usual suspects:

• Alcohol — in 4,098 Finnish employees, HRV-estimated recovery in the first hours of sleep dropped by 9 points with a small dose, and by 39 with a large one (here's what alcohol really does to your night).
• A hard session you haven't absorbed yet — that's normal after intense exercise; it's your body doing its repairs (timelines below; they get shorter as your aerobic fitness improves). If your HRV stays low for several days with no obvious reason, cross-check it with how you feel — here are the signs to watch for.
• Lack of sleep and irregular bedtimes.
• Psychological stress and illness — in healthcare workers wearing an Apple Watch, HRV changed in the 7 days before a Covid-19 diagnosis.

How to increase your HRV over time

There's no quick hack, but there are four documented levers: the amount and regularity of your sleep, regular endurance training, slow breathing at the end of the day — a 2022 meta-analysis (223 studies) shows it increases HRV during the session, right after, and over the weeks — and staying sober the night before important days. After a few months of regular training, your baseline rises: that's one of the signs of improving fitness.

Key takeaways

• HRV measures the variation between heartbeats: high = relaxed nervous system, low = body under pressure.
• Read it against your own baseline, as a trend over several days, measured overnight.
• Alcohol, short nights, training load you haven't absorbed and stress are its four main enemies.