The sympathetic nervous system (SNS) is the 'fight or flight' branch of the autonomic nervous system. When activated, it increases heart rate, dilates pupils, diverts blood to muscles, releases adrenaline and cortisol, suppresses digestion, and heightens alertness. It evolved to help humans survive physical threats — charging predators, falling trees, rival tribes.
The problem: modern life chronically activates the sympathetic system through psychological stressors (emails, deadlines, social media, traffic) that the system wasn't designed to handle. Unlike a charging predator, an overflowing inbox doesn't resolve — so the SNS stays activated. Chronic sympathetic dominance is now implicated in hypertension, anxiety, insomnia, IBS, immune suppression, and accelerated aging.
Breathing is the most direct voluntary control over sympathetic activity. Fast, shallow chest breathing activates the SNS. Slow, deep diaphragmatic breathing deactivates it. This is because respiratory rhythm directly entrains brainstem nuclei that regulate sympathetic outflow. Box breathing, 4-7-8, and coherence breathing all work through this mechanism — they are essentially manual override switches for the fight-or-flight response.