Carefully measured HRV can estimate aspects of cardiac vagal modulation. Slow breathing changes respiratory-linked HRV during practice; durable baseline changes vary and HRV is not a direct measurement of the entire vagus nerve.
A functional description of parasympathetic cardiac modulation. It can be estimated in context, but is not directly read from one wearable score.
Carefully measured HRV can estimate aspects of cardiac vagal modulation. Slow breathing changes respiratory-linked HRV during practice; durable baseline changes vary and HRV is not a direct measurement of the entire vagus nerve.
See also
Learn · Science
Vagal Tone: What It Is and What Breathing Changes
The vagus nerve carries major parasympathetic pathways. “Vagal tone” is inferred from context-dependent cardiac measures; slow breathing changes those measures acutely but does not provide a direct vagus-strength score.
Learn · Science
HRV and Coherence Breathing: What the Research Shows
Heart rate variability rises when the breath is slow, steady, and near the cardiovascular resonance frequency. Here is the mechanism and the evidence.