The autonomic nervous system runs almost everything you don't consciously control — heart rate, digestion, the fight-or-flight sympathetic response, and its counterpart, the calming parasympathetic response. Breathing is the one major exception: it happens automatically, but it can also be consciously overridden. That single fact is what makes breathwork a genuine, physiologically grounded practice rather than purely a relaxation technique.
The mechanism
Specific breathing patterns — particularly those involving extended exhalation relative to inhalation — have a documented effect of stimulating the vagus nerve, the primary channel of the parasympathetic nervous system. Slower, longer exhales are associated in respiratory physiology research with measurable reductions in heart rate and a shift away from sympathetic (stress) dominance toward parasympathetic (rest and digest) activity.
This is why deliberate breathing techniques — used across contexts from clinical anxiety management to elite performance training to contemplative practice — produce real, physiologically measurable effects, independent of any belief in why they work.
Why breath sits at the intersection of the ancient and the clinical
Controlled breathing practices appear across essentially every contemplative tradition with a written record, often thousands of years before autonomic nervous system physiology existed as a field. That convergence — ancient practice, modern mechanism — is one of the more reliably documented overlaps between traditional technique and contemporary physiology.