The problem with population averages
Look up “normal HRV by age” and you will find charts with wide ranges: 20–100 ms SDNN, depending on who you read. These tables average thousands of people of different ages, fitness levels and lifestyles — none of whom are you. A value that looks “average” in a population table can still mean a meaningful shift for your own nervous system.
What a 14-day rolling baseline is
Instead of comparing you to strangers, Tuning compares you to you. It keeps only your own SDNN samples and builds a rolling baseline from the most recent 14 valid days. Each new day, the window slides: yesterday’s sample enters, the sample from 15 days ago leaves, and the baseline gently re-centers on your current reality.
The Day 3 → 7 → 14+ journey
- Day 3 — an initial assessment: the first early signal from your nervous system, not a verdict.
- Day 7 — a formal baseline is established from a full week of your own samples.
- Day 14+ — continuous dynamic adaptation: the sliding window keeps tracking you as life changes.
Why it keeps adapting
Travel, seasons, training load, late nights — your nervous system responds to all of it. A fixed “normal” would become stale within weeks. A 14-day window is short enough to stay current, yet long enough to smooth out single bad days. Your readiness is simply the distance between today’s samples and your personal window.
How Tuning turns this into action
Once the baseline exists, Tuning answers the four daily questions — status, cause, action, outcome — using your real SDNN data. No simulated curves, no population norms: progress is shown only when Apple Health records new real samples.
Key takeaway
Your nervous system has its own signature. A personal 14-day rolling baseline respects that — and it is the only comparison that tells you whether today is a push day or a rest day.