Fight or flight response: definition and how it works

The fight-or-flight response is your nervous system’s automatic reaction to a perceived threat, first named by physiologist Walter Cannon in 1915. Within seconds, your amygdala alerts your hypothalamus, which activates the sympathetic nervous system and releases adrenaline. Heart rate rises, breathing quickens, and blood shifts toward your muscles, preparing you to confront or escape danger.

Where does the term come from?

Harvard physiologist Walter Cannon coined “fight or flight” in his 1915 book Bodily Changes in Pain, Hunger, Fear and Rage, after noticing that strong emotion redirected blood flow and energy away from digestion and toward the muscles needed for survival. More than a century later, his basic model still anchors how researchers describe the acute stress response, even as the details have been filled in by modern neuroscience.

What happens in your body?

A 2018 review in Frontiers in Behavioral Neuroscience traces the pathway. Your amygdala flags a threat and signals the hypothalamus, which fires the sympathetic nervous system. That triggers your adrenal glands to release adrenaline within seconds, and a slower hormonal branch, the HPA axis, follows with cortisol.

Together, these produce familiar signs: a racing heart, fast or shallow breathing, dilated pupils, sweating, tense muscles, and a stomach that goes quiet as digestion takes a back seat. Once the threat passes, the parasympathetic nervous system gradually brings your body back to baseline.

Is fight-or-flight the only stress response?

No. When fighting or fleeing doesn’t feel possible, the nervous system can shift into other automatic patterns instead, like freeze or fawn. Neuroscientist Stephen Porges’ 1995 paper in Psychophysiology, now known as polyvagal theory, first mapped freezing as part of this wider range of nervous-system states; researchers later added fawn as a fourth pattern. Our guide to the freeze response goes deeper on one of them, and all of these patterns start with the same amygdala alarm.

How do you calm a fight-or-flight response?

You can’t out-think an activated sympathetic nervous system, but you can help your body downshift. Slow, longer exhales, grounding through your senses, and movement all cue safety back to your nervous system. Many people also use bilateral stimulation, a left-right rhythm like tapping or eye movements, as a steady daily practice for staying inside their window of tolerance.

EmEase, a self-guided EMDR app, offers the guided version of this practice at app.emease.com. For a step-by-step reset you can use right now, see how to turn off fight-or-flight.