Why do you get butterflies in your stomach?

The sensation comes from the sympathetic nervous system reacting to a perceived challenge, whether a first date, a speech, or a job interview. Adrenaline and related hormones redirect blood flow from the digestive system toward the muscles, heart, and brain, reduce gut motility, and alter the activity of the enteric nervous system, the dense network of neurons lining the digestive tract. The combination of reduced blood flow, changed muscle tone, and altered nerve signaling produces the fluttering, hollow, or queasy feeling.

The gut and brain communicate constantly through the vagus nerve and through hormones and immune signals, which is why emotional states register in the stomach so reliably and why the enteric nervous system is sometimes called the second brain. The same pathway explains why anxiety can cause nausea or an urgent need for the bathroom, why stress worsens irritable bowel symptoms, and why the response is difficult to override by deciding not to be nervous.

Two useful points. The response is not a malfunction and is not correlated with poor performance; it reflects arousal, and moderate arousal improves performance on most tasks, which is why experienced performers describe feeling it before going on and treat it as a sign of readiness. Reframing the feeling as excitement rather than anxiety, which research has tested directly, tends to improve outcomes more than trying to calm down, because it works with the physiology rather than against it. Slow exhalation, which stimulates the vagus nerve, is the one breathing technique with reasonable evidence for reducing the intensity when it is unwelcome.