Vagus Nerve Tone
Your nervous system has a brake. Vagal tone is how hard you can press it.
Grounded in the research on vagal tone
1. Find the brake
The vagus nerve is cranial nerve X, the parasympathetic superhighway running from your brainstem to your heart, lungs and gut. It's the "rest and digest" line. Vagal tone is just how strong and responsive that signal is. High tone = a body that drops into calm fast after stress. Low tone = you stay revved long after the threat is gone. This is the brake pedal on your whole system.
2. Read it with HRV
You can't see the vagus, so you measure its footprint: heart rate variability. Your heart doesn't beat like a metronome. The tiny gaps between beats wobble, and that wobble is the vagus modulating your pulse beat-to-beat. More variability = more vagal control = higher tone. A dead-steady heartbeat sounds healthy but it's the opposite. This is what your Oura ring or Whoop is actually tracking every night.