What does HRV stand for?

HRV stands for heart rate variability: the variation in time between normal successive heartbeats, usually measured in milliseconds. Heart rate counts beats per minute; HRV describes how evenly or unevenly the intervals between those beats occur.

Two people can have the same heart rate and different HRV. An isolated HRV number without its metric, device, timing, recording window, and personal history is therefore difficult to interpret.

What does high or low HRV mean?

Higher or lower is most useful relative to your own comparable baseline. Neither direction is a diagnosis, and a single reading should not be treated as a health verdict.

PatternPossible context—not a diagnosis
Higher than your usual rangeMay accompany recovery, lighter strain, or consistent sleep; higher is not automatically better.
Lower than your usual rangeMay accompany strain, poor sleep, illness, alcohol, or medication effects.
Sudden or inconsistent changeCheck device fit, timing, movement, coverage, and symptoms before interpreting it.

A wearable reading cannot diagnose an arrhythmia, infection, overtraining, or another condition. Symptoms and clinical context matter more than an app score.

RMSSD and SDNN are not interchangeable

HRV is a family of measurements, not one universal number. RMSSD emphasizes short-term beat-to-beat variation, while SDNN summarizes variability across the recording window. A five-minute RMSSD, an overnight reading, and a 24-hour SDNN answer different questions.

A systematic review of portable devices found that agreement with ECG varied by metric, body position, and device method. For a wellness trend, compare the same metric from the same device under similar conditions rather than combining unlike values.

Use a comparable personal baseline

ComparisonUsefulness
One reading vs another personLow: devices, age, physiology, and timing differ.
Several days on the same deviceBetter: look for a sustained change and relevant context.
Recent window vs your prior baselineMost useful for a wellness trend when coverage is adequate.

Keep measurement conditions as consistent as your device allows. Look at sleep, training, alcohol, illness, and data gaps before turning the change into a story.

How Til100 uses HRV today

Til100 treats HRV as a coaching-only factor in Today Signal. It shows the source, recent window, personal baseline, coverage, configured weight, and current model-use status. HRV does not add or subtract lifespan years until a factor-specific model has passed evidence admission, external validation, subgroup, statistical, and safety gates.