Heart Rate Variability (HRV)

Time-domain HRV — SDNN, RMSSD, pNN50 — from beat-to-beat timing. Core metric.

Core metric. SDNN is the headline HRV number.

Definition

Heart rate variability (HRV) is the variation in time between successive heartbeats. Higher variability generally reflects a well-regulated autonomic nervous system and better recovery; chronically low HRV is associated with stress and fatigue. neccessory computes HRV from the beat-to-beat timing of the recovered pulse.

Three time-domain measures are reported:

Metric	Field	Unit	Meaning
SDNN (headline)	`hrv.sdnnMs`	ms	Overall variability across the window.
RMSSD	`hrv.rmssdMs`	ms	Short-term, beat-to-beat variability; the most robust short-term index.
pNN50	`hrv.pnn50Pct`	%	Share of successive intervals differing by more than 50 ms.

Range

Metric	Typical adult range
SDNN	~20–100 ms
RMSSD	~15–90 ms
pNN50	~0–40 %

HRV varies widely by age, fitness, posture, time of day, and breathing.

Averaging window

HRV needs many clean beats. Expect ~15 seconds of good signal before a first estimate; a longer, cleaner capture yields a more stable value. HRV may legitimately be null in a short or noisy session.

Works when

The subject holds still for the full window.
Lighting is even and adequate throughout the capture.
The pulse signal stays clean long enough to accumulate enough beats.

Does not work when

The session is too short or interrupted by motion.
The subject talks, moves, or the lighting flickers.
The signal quality stays low, so too few clean beats accumulate.

Confidence

HRV inherits the top-level signal quality; it has no separate per-metric confidence field. When the contributing signal falls below the reliability floor, HRV values are returned as null. Compare a person against their own baseline rather than population norms.

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