What a readiness reading actually measures
Readiness is not a measure of how fit you are or how hard you trained. It is a measure of the state you woke up in, compared against the state you usually wake up in. Two signals carry it:
- Overnight heart rate variability. The variation between consecutive heartbeats while you sleep, which tends to rise when you are well recovered and fall under accumulated fatigue, illness, alcohol or stress.
- Overnight resting heart rate. Taken from the same night as the HRV reading, so both numbers describe one night rather than two different windows.
Three more signals sit behind them, and they work differently. Sleep duration, respiratory rate and blood oxygen are used only to ask a yes or no question: was this night clearly compromised? A short night or an elevated respiratory rate can pull the day's call down a step. A long night and a low respiratory rate do not push it up, because there is no evidence that more of a good number keeps being better.
Why the comparison is to you, not to a population
A heart rate variability value in isolation says almost nothing. Healthy adults differ from each other by a factor of several, and your own value moves with age, fitness, season and training phase. A number that would be a strong night for one person is a poor night for another.
So the reading is a percentile against your own rolling 60 nights, ending the day before the one being judged. Sixty days is long enough to be a stable reference and short enough to follow you. If you get fitter, the bar moves with you. If you are in a heavy block, the reference knows.
Adjust, hold, accelerate
The reading resolves into one of three calls, shown as a rail with a marker, not as a number:
- Adjust. The night pushed back. Keep the day, lower the ambition.
- Hold. The most common day. Do what you planned.
- Accelerate. Recovery is ahead of your load. There is room if you want it.
The call is not the score alone. Recent training load is folded in, taken from a rolling seven days against your own baseline, so a good night after a very heavy week is read differently from the same night after an easy one. A hard session in the last 48 hours also prevents a push, regardless of how good the number looks.
The phrase is what you read. The number is one tap away, alongside every signal that produced it and where each one sits inside your usual range.
Why it runs on the phone
The whole reading is deterministic arithmetic: percentiles against a stored baseline, a few thresholds, a decision table. There is nothing in it that needs a language model, so there is no reason to send anything anywhere. That has three consequences worth stating plainly. It is instant, it works in airplane mode, and computing it sends nothing off the device.
The written weekly and monthly reports are a different matter, and that page is described separately.
When it stays quiet
A readiness reading is only worth having if it refuses to invent one. VitalBrief will tell you it has nothing rather than fill the gap:
- No night recorded. If the watch was on the charger, there is no reading. Yesterday's is shown as reference, clearly labelled as yesterday's, and never as today's call.
- Thin evidence. A night with only a resting heart rate, or a single stray HRV sample, produces context rather than a verdict.
- Not enough history. Below about two weeks of nights, the card shows its progress instead of a reading, because there is nothing yet to compare against.
The same logic applies to precision. When recent load or a hard session moved the call away from what the raw number alone would say, the marker sits in the middle of its zone rather than claiming a position it cannot justify.
Wellness, not medical. Readiness describes patterns in your own recent data to help you plan training. It does not diagnose, detect or treat anything, and it is not medical advice. If a signal concerns you, talk to a clinician.
