During stress, adrenaline dilates your pupils to sharpen your vision and help you detect threats faster.

Pupil dilation during stress is called mydriasis and is controlled by the sympathetic nervous system. When your amygdala detects a threat, it signals your hypothalamus to release adrenaline from the adrenal glands. This hormone acts on the dilator pupillae muscles in your iris, causing them to contract and pull your pupil open.Research published in Psychophysiology shows that pupil size is a reliable indicator of cognitive load and emotional arousal. By widening the eye's aperture, your body maximizes the amount of visual information reaching your retina. This biological adjustment improves visual acuity in low-light environments or high-stakes survival situations.However, this intense focus often comes with a trade-off called "tunnel vision." According to the American Psychological Association, your brain prioritizes central vision to track a specific threat, which can reduce peripheral awareness by up to 70 percent. This physiological shift was extensively documented in combat stress studies throughout the 20th century.Modern optometry confirms that even non-physical stressors—such as solving a difficult math problem or hearing a sudden loud noise—can trigger this response. The dilation occurs within milliseconds of the adrenaline surge, making it one of the fastest autonomic reactions in the human body. This mechanism remains a vestigial survival trait from our evolutionary past.