HRV: The signal your doctor ignores

Your resting heart rate gets a number on your Apple Watch. Your heart rate variability gets nothing — or worse, a single meaningless daily score that swings wildly and tells you nothing actionable.

That’s a problem. HRV is the single best non-invasive window into your autonomic nervous system, and almost no one reads it correctly.

What HRV actually measures

HRV is the variation in time between consecutive heartbeats. Not the average — the variation. A heart beating at 60 bpm isn’t ticking like a metronome at exactly 1,000 ms per beat. It fluctuates: 980 ms, 1,020 ms, 1,040 ms, 995 ms. High variability means your nervous system is responsive and adaptive. Low variability means it’s locked in a stress state.

The metric that matters is rMSSD: root mean square of successive differences. It’s the least noisy HRV measure for overnight passive recording.

The baseline problem

Here’s where consumer wearables fail you. They show you a raw rMSSD number and compare it to population averages. But HRV is deeply individual. A 38 ms rMSSD might be excellent for one person and a burnout signal for another.

What you need is a personal baseline — your own 90-day rolling average — and a deviation signal. A 15% drop below your baseline on consecutive mornings is a flag. A single low reading after a late dinner and two glasses of wine is noise.

Vitaeon tracks your personal baseline from day one and only surfaces deviations that matter.

When HRV predicts illness

A 2023 systematic review of wearable HRV studies found that both rMSSD and SDNN fall before COVID-19 symptoms appear — SDNN reductions showing up as early as seven days ahead of a positive test. Your immune system activating shifts autonomic balance before you feel anything. The signal is there. You just need something tracking it continuously.

Two caveats from that same review are worth stating plainly. The evidence is concentrated in respiratory infection, mostly COVID — not illness in general. And because HRV is so individual, the reviewers could not put a number on how far a decline has to go before it means something. There is no validated threshold to quote. Which is precisely why the baseline argument above is the whole game.

The pattern Vitaeon watches for: three consecutive mornings where rMSSD is more than 12% below your 90-day average, without an obvious explanation like travel, alcohol, or a hard training block. That threshold is ours, not the literature’s — a starting point chosen to surface a real pattern without crying wolf, and one we will move as we see more data.

What to actually do with it

HRV is not a performance metric to optimize obsessively. It’s a readiness signal.

The bands below are the ones Vitaeon uses. They are calibrated judgement, not published cutoffs — no such cutoffs exist, for the reason given above.

• 8–15% below baseline: Reduce training intensity today. Prioritize sleep and hydration.
• 15%+ below baseline for 3+ days: Rule out accumulating stress, poor sleep quality (check deep sleep duration), or early illness. Consider rest rather than active recovery.
• Trending up over 30 days: Your recovery protocols are working. Don’t change anything.

The goal isn’t the highest number. It’s a stable, gradually rising baseline over months — which tracks closely with improved fitness, stress management, and sleep quality.

The confounders nobody mentions

Alcohol suppresses HRV and raises resting heart rate overnight, and it does so dose-dependently: an analysis of five million person-days of wearable data found the effect at intakes as low as a single drink, with larger changes in women and in younger adults. One late drink can tank tomorrow morning’s reading even if you sleep 8 hours. Track it alongside your HRV data and you’ll see your own pattern within two weeks.

Late eating (within 2 hours of sleep) has a similar but smaller effect. Your body is still digesting rather than recovering.

Altitude, travel across time zones, and high ambient temperatures all suppress HRV temporarily. These aren’t failure states — they’re context. Good tracking tools label them so you don’t misinterpret signal as noise.

Aeon surfaces your personal HRV trend weekly, not daily, and flags deviations in plain language: “Your HRV has been 18% below your 90-day average for 4 days — likely from the travel Wednesday. Prioritize sleep this weekend before Monday’s training session.”

Number first. One action. No dashboard to interpret.