Key takeaways
- 1The vagus nerve is the main parasympathetic highway.
- 2Vagal tone is the strength of that signal — measured non-invasively via HRV.
- 3Higher vagal tone correlates with resilience, recovery, and lower resting arousal.
- 4Slow breathing exercises the vagal circuit and, practiced consistently, raises resting vagal tone.
What the vagus nerve does
The vagus is the tenth cranial nerve. It carries parasympathetic signals from the brainstem to the heart, lungs, and digestive tract. It also carries afferent signals from those organs back to the brain — most of what the brain "knows" about your gut and internal state comes through the vagus.
Vagal tone as a training target
Vagal tone reflects the strength of the parasympathetic signal at rest. It is estimated from HRV — specifically the RMSSD or high-frequency components. Higher tone means more parasympathetic dominance at rest, which correlates with recovery capacity, stress resilience, and healthy aging markers.
How slow breathing trains it
Every slow, exhale-emphasised breath fires the vagus. Do that consistently for weeks and the vagal circuit — like any circuit — strengthens. Resting vagal tone climbs. This is what makes coherence breathing worth doing daily rather than occasionally.
The polyvagal caveat
Stephen Porges\' polyvagal theory has been influential in trauma-informed practice. Parts of the theory remain debated in mainstream neuroscience. The functional practice — slow breathing raises vagal tone — holds up regardless of which theoretical frame you prefer.