HRV — what it is, and why the number alone means little

Heart-rate variability is the variation in the time between consecutive heartbeats. It reflects the balance of your autonomic nervous system: when you are rested and recovered, the parasympathetic ("rest and digest") side dominates and the spacing between beats varies more. Under load — hard training, poor sleep, illness, alcohol, life stress — that variation narrows.

Your absolute HRV number is not comparable to anyone else's. This is the single most important thing to understand about it, and the reason "is 107 ms good?" has no useful answer on its own. HRV depends on:

• Age and genetics — healthy adults span roughly 20 ms to 200 ms, and much of that spread is simply who you are, not how fit you are.
• How it was calculated — RMSSD, SDNN and lnRMSSD are different maths on the same heartbeats and give different numbers. Devices don't agree with each other.
• When and how it was measured — an overnight average and a reading taken on waking are different measurements. Posture, breathing rate and even whether you'd been awake five minutes all move it.

So a runner sitting at 45 ms is not less recovered than one at 110 ms. Comparing your number to a friend's, or to a "normal range" from the internet, tells you nothing.

What does mean something is your own trend. Runsei compares your recent HRV against *your* established normal — a 30-day median that deliberately excludes the most recent week, so a hard training block cannot quietly redefine what "normal" is for you. Above your normal generally means well recovered; below it, especially for several days running, means the load is outweighing the recovery.

Read it with the other signals, never alone:

• HRV down + resting HR up is the clearest "back off" pattern there is.
• HRV down the day after a hard session is expected, not alarming — that is training working. Runsei checks whether a quality session in the last few days explains a dip before it says anything about it.
• Unusually high HRV alongside heavy fatigue and poor performance is worth noticing rather than celebrating; deep fatigue can occasionally present that way, which is exactly why one number never decides anything.

Consistency is what makes it usable. Measure the same way each time — same method, same time of day, same posture. If you log by hand, tell Runsei which basis you use under Profile → Morning check-in & readiness so your entries stay comparable with each other. A single reading is noisy; the pattern over a week is the signal.