A healthy heart does not beat like a metronome. The interval between one beat and the next is constantly changing — by a few milliseconds, hundreds of times an hour. Heart-rate variability is the measurement of that change.
Counter-intuitively, more variation is generally the better state. A heart that is free to fluctuate is a heart that is not being held under tight control.
What it actually reflects
HRV is the clearest non-invasive window onto the autonomic nervous system — the part of you that decides between effort and recovery without consulting you. Its two branches pull in opposite directions: the sympathetic branch raises heart rate and steadies its rhythm for action, the parasympathetic branch lowers it and lets the rhythm breathe.
When you are rested, the parasympathetic branch has room to work and variability rises. Illness, alcohol, a short night, a hard session or sustained stress all push the balance the other way, and variability falls — often before you notice anything yourself.
Why only your own number matters
Absolute HRV varies enormously between people. It falls with age, differs by measurement method, and two healthy adults of the same age can sit a long way apart. A number that is normal for you may be high or low for someone else, which makes comparison between people close to meaningless.
The comparison that carries information is tonight against your own trailing average. That is the only comparison Aurora makes.
How Aurora reads it
From the beat-to-beat series produced by the pulse sensor, taken during the most settled part of the night — when posture, breathing and room conditions are as close to constant as they get. Measuring at the same point each night is what makes one night comparable to the next.
What it cannot tell you
A change in HRV tells you that something moved. It does not tell you what. A late meal, a glass of wine, a warm room, a genuine infection and a hard training session can all push it in the same direction.
It is not a diagnosis and it is not a warning system. Treat it as one input into a decision you were going to make anyway.