What is heart rate variability, and are wearables useful?
Heart rate variability (HRV) is the variation in time between consecutive heartbeats — and counterintuitively, more variation is generally better, because it reflects a nervous system responding flexibly rather than one stuck in a fixed state.
What it reflects. The balance between the two branches of the autonomic nervous system: the sympathetic (stress, activation) and parasympathetic (rest, recovery) systems. A well-recovered, relaxed state tends to show higher variability; stress, illness, poor sleep, alcohol and heavy training tend to suppress it.
How it is measured. From the intervals between heartbeats. RMSSD is the most common short-term metric, alongside SDNN and various frequency-domain measures. A chest strap measuring the electrical signal is more accurate than a wrist optical sensor, particularly during movement.
The single most important caveat: absolute values are almost meaningless between people. HRV varies enormously with age, genetics, fitness and measurement method. Comparing your number with someone else's tells you nothing. Only your own trend, measured consistently, is informative — same time of day, same posture, ideally on waking.
What a suppressed HRV can indicate: inadequate recovery, accumulating training load, poor sleep, alcohol the night before — which shows up very reliably — illness, including before symptoms appear, and psychological stress, which the measure does not distinguish from physical stress.
Where wearables are genuinely useful:
Trends over weeks, which are more reliable than any single reading.
Detecting illness early, where a sharp drop frequently precedes symptoms.
Showing the effect of alcohol and poor sleep concretely, which changes behaviour more effectively than advice does.
Adjusting training when readings are persistently down.
Where they are weaker:
Day-to-day noise is large. A single low reading means little, and reacting to each one is a mistake.
Composite "readiness" scores are proprietary, unvalidated and combine inputs opaquely.
Anxiety about the number is a documented phenomenon — worrying about a low score is itself a stressor, and for some people tracking is net negative.
Consumer devices are not medical instruments. Report persistent unexplained changes to a doctor rather than self-interpreting.
General information, not medical advice.