The Autonomic Nervous System and Breathing: Sympathetic, Parasympathetic and What Breathing Changes

What the autonomic nervous system is

The autonomic nervous system, also called the involuntary nervous system, runs body functions you do not have to think about. NHLBI notes that breathing is usually controlled by it. [1] It has two main branches. The sympathetic system increases heart rate, blood pressure, breathing rate and pupil size. The parasympathetic system slows the heart, relaxes digestive muscles and, for the airways, slows breathing rate and narrows the bronchial tubes. [1]

Breathing is unusual because it is both automatic and voluntary. NHLBI notes you can control your breathing pattern at times, for example when you sing or hold your breath. [1] That is the doorway through which breathing practices are thought to reach the rest of the body. This page is information only and not medical advice.

What slow breathing changes

Several measurable things happen when healthy people breathe slowly on purpose:

• Heart rate variability (HRV). HRV is the natural beat-to-beat change in heart rate. A systematic review and meta-analysis of 223 studies found increases in vagally-mediated HRV during voluntary slow breathing, right after one session, and after multi-session programs. [2] Its authors say voluntary slow breathing is "suggested" to influence the parasympathetic nervous system and conclude it could be advised as a low-tech technique with few adverse effects expected. [2]
• Respiratory sinus arrhythmia (RSA). This is the rise and fall of heart rate with each breath. A 2018 systematic review of 15 studies reported that slow breathing increased HRV and RSA along with changes in brain activity, and reports of more comfort and relaxation. [3]
• Other systems. A 2017 review described effects of slow breathing in healthy people on respiratory muscle activity, ventilation efficiency, baroreflex and chemoreflex sensitivity, blood flow and sympathovagal balance, and called for further research. [4]
• Longer exhalations. A small scoping review of six studies in healthy adults reported that RSA rose significantly in studies using tailored breathing patterns, especially ones emphasizing longer exhalations. [5] Six studies is a small base; see extended-exhale breathing.

All of these are physiological findings. They are not the same as health outcomes.

How it might work: proposed mechanisms

These are proposals, not established facts.

• The baroreflex. A review of heart rate variability biofeedback, a method that guides slow breathing, called strengthening of the baroreceptor reflex (a blood pressure control loop) the most supported possible mechanism, and also noted a proposed effect on vagal afferent pathways to the frontal cortex. [6]
• Respiratory vagal stimulation. A theoretical review proposed that regulated or attentively guided breathing in practices such as meditation and yoga acts by phasically and tonically stimulating the vagus nerve. [7] It labels this a model, with limitations.
• Body sensing and nasal airflow. The 2018 systematic review offered two hypotheses, one about sensing internal body states and one about nasal airflow receptors affecting the brain's olfactory activity. [3]

Where the simple story breaks

"Slow breathing calms your vagus nerve and switches you into rest mode" is common online. The sources we read support something narrower.

1. HRV is an indirect measure. A review of respiratory sinus arrhythmia listed caveats about treating it as a direct measure of vagal tone: breathing itself can confound the relation, and the measure is influenced by physical activity and other factors. [8] A methods paper warns HRV findings are easily misconstrued. [9]
2. Foundations are debated. A 2025 review said recent findings have challenged several foundational concepts traditionally believed to underlie the effects of breathwork. [10]
3. Physiology is not outcome. NCCIH notes that trials of slow breathing and blood pressure did not look at stroke or heart attack, and had differing methods, short follow-up and high risk of bias. [11]
4. "More HRV" is not "more health". For devices and scores see HRV biofeedback and vagus nerve claims.

We found proper evidence not found for the claim that any one breathing technique reliably "resets" the nervous system in people with anxiety, stress or illness; our searches covered PubMed reviews on slow breathing and HRV and NCCIH and NHLBI pages.

Evidence summary