Cortisol: the stress hormone, explained simply

Cortisol is your body’s main stress hormone, made by your adrenal glands and controlled by the hypothalamic-pituitary-adrenal (HPA) axis. When your brain flags a demand or threat, this axis releases cortisol to raise blood sugar, sharpen alertness, and fuel the fight-or-flight response. Levels also rise and fall on a daily rhythm, separate from short stress spikes.

What does cortisol actually do?

Cortisol gets called “the stress hormone,” but that undersells it. A StatPearls physiology review describes it as central to blood sugar, metabolism, blood pressure, and immune activity, not just moments of stress. It’s made in the outer layer of your adrenal glands, two small glands above your kidneys, after your hypothalamus and pituitary gland trigger its release.

Cleveland Clinic notes cortisol affects almost every organ and tissue in the body, part of why too much or too little can throw several systems off at once. During an acute stressor, cortisol works alongside adrenaline as part of the fight-or-flight response. That cascade typically starts when your amygdala flags something as a threat.

Why does cortisol rise and fall through the day?

Outside of acute stress, cortisol follows its own daily rhythm. Per StatPearls, it typically peaks around 8 a.m. to help you feel alert and awake, then declines gradually until it hits its lowest point between midnight and 4 a.m.

That’s one reason a single cortisol test rarely tells the full story: a level that’s normal at 9 p.m. would look unusually high at 3 a.m. Clinicians often need multiple samples at different times of day to get a meaningful read.

What happens when cortisol stays elevated?

Cortisol is meant to rise, then fall again once a stressor passes, completing what’s sometimes called the stress response cycle. Neuroscientist Bruce McEwen’s concept of “allostatic load” describes what happens when stress systems like this one stay switched on too often, or for too long. He frames it as a kind of cumulative wear on the body. That wear can show up as disrupted sleep, a low mood, or trouble concentrating, well before anything shows up on a lab test.

Feeling keyed up, easily startled, or unable to power down are common signs of a nervous system stuck in overdrive. Our hyperarousal entry covers what that looks like day to day, and our guide to stress and burnout goes deeper on what helps.

Where does bilateral stimulation fit in?

Cortisol is one piece of the same stress-response system behind a racing heart or a tight chest. Most bilateral-stimulation research measures calming shifts in heart-rate variability and breathing, not cortisol directly, but it targets that same overall system. Bilateral stimulation is the left-right rhythm at the center of EMDR. EmEase, a self-guided EMDR app, offers a guided version of this practice for everyday stress at app.emease.com.