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If you take one idea from this site, take this one. A single health measurement tells you almost nothing. The same measurement, seen over weeks, tells you a great deal.

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An example you can feel

Heart rate variability — HRV — is the variation in time between your heartbeats. Higher generally means a body that is coping well; lower generally means one under load. It is one of the more useful numbers a wearable can produce. Now, here is what moves your HRV reading on any given night:
  • How much you slept, and when
  • Whether you drank alcohol, and how much
  • Whether you are fighting something off
  • Your last meal, and how late it was
  • Hard training in the last 48 hours
  • The altitude you slept at
  • How tight the strap was
A single night’s HRV is all of those things mixed together. Read on its own it is close to meaningless, and reacting to it is worse than ignoring it. Three weeks of nights is a different object entirely. The strap tightness averages out. The one bad meal disappears. What is left is signal — and if it moves, something real moved.

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What that means on screen

Several things in the app follow directly from this.
Most figures need a baseline before they mean anything, and a baseline needs nights. Some need two weeks. This is not the app warming up — it is the app declining to tell you something it does not yet know.
Where a figure would be misleading without context, you will see a state — a band, a direction, or nothing at all — rather than a falsely precise number.
If your Capacity drops eight points overnight, the useful question is not “what did I do wrong yesterday” but “is this still true in four days”.

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The one exception worth knowing

Trends need comparable days. If you switch which watch you wear halfway through a month, the trend breaks — not because anything happened to you, but because the instrument changed underneath the measurement. That is why the app asks you to nominate which device it should trust for what, and why changing it later is worth doing deliberately rather than casually. See Choosing your sources.