Not just fight or flight: how stress affects your body and brain

You’re walking home late at night when you suddenly spot a silhouette up ahead. Immediately, you experience an emotional response: fear. But a split second before you become conscious of your fear, your body has already launched the chemical reaction we call “stress.”

It all starts in the amygdala, the two-part region in the temporal lobes of your brain, which monitors all external stimuli, trying to identify any information that could be dangerous to your life or health. 

The amygdala instantly juxtaposes signals from the sensory organs (the silhouette in the dark) with memories of similar situations in the past ( a friend of yours was mugged walking late at night) and general knowledge (this neighborhood isn’t safe after dark), concluding that you’re in danger. The alarm signals are then sent to the diencephalon, from which the hypothalamus launches the ancient “fight or flight” response. 

This releases hormones into your body to put it into battle readiness. Adrenaline speeds up your pulse and breathing, and blood starts pumping to your tensed muscles. To maximize your energy reserves for this incoming threat, cortisol signals glucose to be released into the bloodstream, while your immune response, anti-inflammatory processes, and digestion are all slowed down.

Stress fills us with strength and endurance to give us a better shot at surviving a dangerous situation. But this is a very energy-consuming process, and your body isn’t designed to be able to handle it for prolonged periods. In a perfect world, your body would return to its initial relaxed state as soon as you realize that the silhouette was just a cat crossing the street. But in the real world, stress can linger for a long time, rising and falling but never fully subsiding since we’re constantly surrounded by stressors. Between work, family arguments, financial difficulties, or existential fears, these stressors aren’t as easy to dismiss as a momentary scare in a dark alley.

On the physiological level, chronic and acute stress responses look the same. We generally don’t notice this because the chronic stress response is not as intense: your breathing is shallow but not as elevated; your muscles are tensed, but not as much, like a tighter jaw and raised tight shoulders. Still, chronic stress uses up an enormous amount of resources, and important organ systems suffer as a result.