Stress and the brain: what cortisol does, and 5 habits that calm it

An exam, an argument, a day that spills over: within seconds, your brain sounds the alarm. That reaction protects you. But when it never switches off, it weighs on memory, focus and mood. Here is what actually happens inside your head, and five simple habits whose effect has been measured.

What happens in your brain when you are stressed

Stress starts in the head. Your brain is what decides whether something is a threat: an urgent email, a noise at night, a talk you have to give. The neuroscientist Bruce McEwen put it simply: the brain is the central organ of stress. It interprets what is going on, then it tells the body how to respond.

It all begins in a small almond-shaped structure, the amygdala, which spots anything that looks like danger. It alerts the hypothalamus, which launches two systems, one after the other:

• Within seconds, the nervous system triggers the release of adrenaline. Your heart beats faster, you breathe faster, your muscles get ready.
• Within minutes, a chain of hormones runs from the hypothalamus to the pituitary gland, then to the adrenal glands, which release cortisol. It raises blood sugar to give you energy, and it travels back to the brain.

Cortisol is not the enemy. It follows a daily rhythm: high in the morning to get you going, low in the evening to let sleep come. Stress adds peaks on top of that rhythm. Normally, once the danger has passed, cortisol itself puts the brakes on the system and the response fades.

Short bursts of stress help, up to a point

Brief, moderate stress sharpens attention and boosts energy. It is what makes you hit a deadline or brake in time. But one region copes badly with pressure: the prefrontal cortex, just behind your forehead. It holds information in mind, makes plans and keeps impulsive reactions in check.

In 2009, the neuroscientist Amy Arnsten brought the research on this together. Even fairly mild acute stress, when you feel you have no control over the situation, can cause a rapid and dramatic loss of prefrontal abilities. That is the blank mind during an exam, or the sentence that will not come in a meeting. The brain hands over to faster, more automatic reactions: useful in real danger, far less useful for thinking.

When stress drags on: memory, emotions, focus

The problem is not the spike of stress. It is the stress that never comes down. A 2009 review in Nature Reviews Neuroscience (Sonia Lupien and colleagues) points out that three regions are rich in cortisol receptors, and therefore sensitive to prolonged stress:

• the hippocampus, essential for forming new memories;
• the amygdala, the alarm detector, which becomes more reactive;
• the prefrontal cortex, which organises thinking and calms emotions down.

In animals, repeated stress shrinks the branches of neurons in the hippocampus and the prefrontal cortex, while those in the amygdala grow. The brain becomes quicker to raise the alarm and worse at calming itself. The good news: in these studies, much of this change reverses once the stress stops. The brain keeps its ability to reshape itself.

Anxiety vs stress? They feel alike, but they are not the same thing. Stress is a response to a pressure you can name: a deadline, a conflict, an overload. Anxiety is worry that lingers, even when the cause is vague or already gone. If you worry all the time without really knowing why, an anxiety questionnaire can help you take stock (for now, our questionnaire is in French).

5 calming habits, and what the research says

1. Breathe slowly. Slowing down to fewer than ten breaths a minute is the fastest tool you have. A 2018 review (Andrea Zaccaro and colleagues) kept 15 studies of healthy people. Slow breathing increases heart rate variability, a sign that the body's calming system is taking over again. Participants reported feeling more relaxed and less anxious. In practice: breathe in for 5 seconds, out for 5 seconds, for 5 minutes. This is sometimes called coherent breathing.

2. Move. A very large 2023 overview (Ben Singh and colleagues) pooled 97 reviews, covering 1,039 trials and more than 128,000 participants. Physical activity had a medium-sized effect on depression, anxiety and psychological distress, and more intense activity brought bigger improvements. A brisk walk, the stairs, a short bike ride: it all counts.

3. Spend 20 to 30 minutes in nature. Researchers at the University of Michigan asked 36 city dwellers to spend at least 10 minutes somewhere that felt like nature, three times a week, for eight weeks. They measured cortisol in saliva before and after. Cortisol fell by 21% per hour on top of its natural decline over the day. The best return on time spent came between 20 and 30 minutes. A park is enough.

4. Talk to someone you trust. In a 2003 experiment in Zurich, 37 men had to speak in public in front of a panel, a very stressful task. Those who had been supported by their best friend while preparing produced less cortisol. A call, a coffee, a few words: support from people close to you really does dampen the stress response.

5. Meditate, with realistic expectations. A 2014 review (Madhav Goyal and colleagues, 47 trials, 3,515 participants) found that mindfulness meditation slightly reduces anxiety and depression. For stress itself, the evidence is weaker. And meditation did no better than other active approaches such as exercise. It is one tool among others: useful if you enjoy it, not essential if you do not.

What matters most is doing it regularly: a few minutes every day beat a long session now and then. And sleep remains the foundation, because a lack of sleep makes you more sensitive to stress.

Check where your stress stands

Before changing your habits, it helps to know where you are starting from. Our stress and well-being test has 16 statements and takes about 4 minutes. It builds on the work of Sheldon Cohen, whose Perceived Stress Scale is widely used in research. There is no sign-up. At the end, you see your stress zone and your main lever for change. The test is in French for now.

An online test helps you take stock. It does not replace medical advice. If stress has lasted several weeks, if it is damaging your sleep, appetite, work or relationships, or if you feel overwhelmed, talk to your doctor or a psychologist. Asking for help early is often the quickest way to feel better.

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Sources

The MyMindTests team writes about the brain from published studies. Every figure is sourced and checked before publication.