Parasympathetic nervous system

The parasympathetic branch dominates when the body is at rest: heart rate slows, digestion resumes, and the processes of repair and storage take priority. Most of its output travels through the vagus nerve, and its neurotransmitter is acetylcholine.

It matters because recovery is an active state, not merely the absence of training. Parasympathetic activity is what allows sleep, digestion and tissue repair to proceed properly, and it is the main contributor to the beat-to-beat variability measured as HRV.

In practice it can be nudged deliberately. Slow breathing with a long exhale, particularly at around six breaths a minute, reliably increases parasympathetic activity — one of the few recovery interventions with a clear mechanism and immediate measurable effect.