Lately I’ve been thinking a lot about stress, how everyone around me seems one email away from a breakdown and how my own life has become a perfectly timed sequence of atomic meltdowns every six hours. So let’s revisit the question: what actually happens in our brains when we’re stressed? I don’t mean that mild panic when you realize you forgot someone’s birthday, but the kind that leaves you frozen. And most of all: is cortisol really the villain everyone says it is?
People love to blame cortisol for everything: your insomnia, your overreactions, even your questionable culinary choices. The usual story is simple: cortisol levels rise, your brain melts down, and you become a frazzled version of yourself. But that’s not quite true. Cortisol isn’t the problem: it’s part of a system that was built to keep you alive. The real issue is what happens when that system never shuts off.
What is cortisol? Cortisol is a steroid hormone produced by the adrenal glands in response to stress. It helps regulate metabolism, blood sugar levels, and the immune response, ensuring the body can react quickly to challenges. In short bursts, it sharpens focus and boosts energy. The problem arises when cortisol stays elevated for too long: what’s meant to be a survival mechanism turns into a slow, steady drain on your brain and body.
The Hippocampus and Cortisol
At the center of it all is the hippocampus, the region responsible for turning short-term experiences into long-term memories. When it’s working well, you learn, adapt, and remember. When it starts to fail, everything becomes a blur. One of the earliest signs of Alzheimer’s is repetitive questioning: the hippocampus has stopped sending new information to long-term storage. Under chronic stress, something similar takes place. Memory falters, focus slips, and the ability to stay calm begins to vanish.
The hippocampus is full of cortisol receptors, molecular locks designed precisely for that hormone. In normal circumstances, cortisol helps manage energy, alertness, and recovery. But when stress becomes a daily default, those receptors are bombarded. Over time, the cells begin to alter how they express certain genes, changing the brain’s internal wiring and, in some cases, leading to physical damage in hippocampal tissue.
Cortisol, Corticosterone, and the Lab Rats
Much of what we know about this comes from animal studies. In humans, the main stress hormone is cortisol, while in rats it’s corticosterone. The mechanisms are so similar that researchers often use them interchangeably when studying stress. Among the most famous were the experiments by neuroscientists Bruce McEwen and Elizabeth Gould. They subjected rats to different stressful experiences and then examined the effects on their brain structure and behavior.
What they found turned the traditional idea of stress upside down. It wasn’t simply the amount of stress hormone that determined the outcome: it was the nature of the experience itself. When the stress was uncontrollable, such as when rats were restrained and terrified, the consequences were clearly harmful. Their dendrites, the delicate branches that connect neurons, began to shrink. The animals’ ability to learn declined, and they displayed depressive, withdrawn behavior. But when the stress came from voluntary activity, such as running on a wheel, the same hormone surge produced the opposite effect. The dendrites grew, neural connectivity increased, and the rats became more curious and active.
In other words, the hormone wasn’t inherently toxic. It was a tool, and the outcome depended entirely on how the organism perceived the situation. If the experience was one of control and purpose, the brain adapted and grew stronger. If it was one of fear and helplessness, the same chemical cascade became destructive.
Context is more important than chemistry
This is the paradox of stress: context matters more than chemistry. Psychologists sometimes call this adaptive versus toxic stress. Adaptive stress pushes the system just enough to grow: it’s like lifting weights for your neurons. Toxic stress traps you in a loop of alarm and exhaustion, slowly eroding memory, motivation, and immune function.
Cortisol itself isn’t bad; it’s a messenger. What determines whether it builds you up or breaks you down is the message your brain hears when it arrives. Somewhere deeper in the brain, another structure is shaping that message: the amygdala. And that’s where the real chaos begins.
This article is Part One of the English version of the original piece (in Spanish), available here.