Vagal tone refers to the activity of the vagus nerve — the parasympathetic 'rest and digest' counterweight to sympathetic 'fight or flight.'
Mechanism — Vagal activity slows heart rate, supports digestion, dampens inflammation (via the cholinergic anti-inflammatory pathway), and regulates emotion. Measured indirectly via HRV (RMSSD specifically).
Use case — Improve via slow nasal breathing, cold exposure, meditation, singing/humming, sufficient sleep, and aerobic training. Polyvagal-informed therapies emphasize this lever.
Caveats — Vagal nerve stimulators (medical devices) are different from 'vagal toning' practices. The therapeutic devices are FDA-approved for specific conditions. The practices are supportive lifestyle inputs.