Heart rate variability appears on wearables as a readiness or recovery figure. The underlying measurement is well defined, and understanding it clarifies why the daily number moves so much.
The measure is beat-to-beat variation
A healthy heart does not beat at perfectly even intervals, and heart rate variability quantifies the differences between consecutive intervals rather than the average rate.
Those differences arise because the heart receives continuous input from two branches of the autonomic nervous system, one accelerating and one slowing it.
Variability is therefore read as an indirect window onto autonomic balance, which is why it draws interest in both athletic and clinical contexts.
Breathing drives much of it
Heart rate rises slightly on inhalation and falls on exhalation, a pattern that accounts for a large portion of the variability measured over short periods.
Because breathing rate and depth strongly influence the result, a slow deliberate breathing pattern during a measurement raises the number substantially.
This is worth knowing before interpreting a device reading taken while a person was consciously relaxing, since the measurement partly reflects the breathing.
Measurement conditions dominate the result
Values differ depending on posture, time of day, recent food, alcohol, temperature and whether the measurement lasted seconds or hours.
Comparisons are only meaningful when conditions are held constant, which is why devices measure during sleep or immediately on waking.
Different metrics also exist, computed in the time domain or the frequency domain, and vendors rarely state which one their displayed score is based on.
Between-person comparison is nearly meaningless
Absolute values vary enormously between individuals for reasons including age, genetics and baseline fitness, so one person's number tells nothing about another's.
The informative signal is the deviation from a person's own established baseline, which requires weeks of consistent measurement to establish.
Devices attempting a single readiness score compress all of this into one figure, and the compression is where most of the interpretation problems begin.
What a low reading means and does not
A reading below baseline indicates the autonomic state differed that morning, which can follow hard training, poor sleep, illness, alcohol or emotional strain.
It does not identify which of those occurred, and the number cannot distinguish productive training stress from a developing infection.
Heart rate variability is used in clinical research but is not a diagnostic test, and symptoms such as palpitations, dizziness or chest discomfort require medical evaluation.