Most of us would describe stress with feelings such as anxiety, frustration, irritability, fear, or worry. We’d say stress can make us feel short-tempered. We may snap at a coworker, get impatient with a loved one, or lie awake at night playing through all sorts of scenarios that may or may not happen. All of these things are mental and emotional states that come from our brain, and because of this, we often think of stress as a cognitive event that happens in our head.

But the reality is that stress is a significant physical event that begins with the brain.

Once our brain registers that a stressful event is happening or has the potential to happen, it sends chemical and electrical messages to the rest of our body. This results in a release of many different categories of hormones that produce very quick and dramatic changes to our entire chemical makeup and affect all types of systems in the body. These changes influence brain function, energy, metabolism, appetite, sleep, immunity, and more.

Stress is a chemistry problem.

The stress hormones adrenaline, noradrenaline, and cortisol signal rapid changes to the entire body. They are all good changes, perfectly designed to help us meet the challenges of stress—so long as we follow through with the next step of the cycle. We do that when we fight or flee, or more realistically in today’s world, when we Play It Out. That’s the good news. But there’s a lot of bad news about these stress hormones when we don’t hold up our end of the deal.