Your Brain on Stress

In plain words

When something feels threatening, demanding, unpredictable or out of your control, your brain gets your whole body ready to deal with it. Two systems do most of this work.

The first one is fast. It’s called the sympathetic nervous system, and within seconds it makes you more alert, speeds up your heart and gets more energy ready.

The second one is slower, and it lasts longer. It’s called the HPA axis. A message passes down a chain, from the hypothalamus in your brain to the pituitary gland, just under the brain, and then to the adrenal glands above your kidneys, which release a hormone called cortisol. Cortisol has a bit of a villain reputation, but cortisol isn’t bad. You need it! It helps run your metabolism, your immune system, your heart and blood vessels, and your energy when life gets demanding.

Stress turns into a problem when these systems keep switching on, or stay on for a long time, without enough chance to recover.

What a moment means to you matters as much as the moment itself. A deadline, an argument with someone you love, money worries, pain that won’t go away, a bad night’s sleep, being left out and real physical danger all look very different from the outside. They can still switch on many of the same stress systems inside you.

Under stress, your brain changes its priorities, and a lot of the time that helps. A bit of stress can sharpen you before a big presentation. Or say a car horn blares while you’re crossing the road. You don’t want your brain calmly weighing up seventeen possible interpretations. You want it to get you out of the way! Research on stress and thinking skills finds that the effect depends a lot on how strong the stress is, when it hits, what you’re trying to do and who you are.

In plain words

The front of your brain, the prefrontal cortex, does much of what you rely on to be a reasonably functional adult. It holds things in mind, keeps you focused, stops you acting on every impulse, makes plans and helps you switch from one idea to another.

Under a lot of stress, some of this can work less well, especially holding things in mind and thinking flexibly. The effect on self-control is more mixed.

That’s why you can suddenly struggle with things you normally handle, like answering an email or remembering what you went to the supermarket for. When you’re stressed, it can feel as if your brain has temporarily misplaced its fanciest features. Your skills are all still there. Stress makes them harder to reach in the moment.

Under sudden stress, the quick parts of your brain that spot threats and run habits get more say, and slow, careful thinking takes a back seat. That helps in an emergency. Modern life is full of moments where the better move is to sit down, think it through and send a sensible email. Your brain doesn’t always tell those situations apart as neatly as we’d like.

Then there’s the amygdala, a small almond-shaped part of the brain (fun bit of trivia: “amygdala” comes from the Greek word for almond). It’s often called the brain’s “fear center,” and that’s only part of the story. It helps you notice and learn from things that matter emotionally, especially anything that feels threatening or uncertain. Under stress, emotional things can stand out more, which shapes what you notice and remember.

It doesn’t just take over, though. The amygdala is always in conversation with other parts of the brain, including the prefrontal cortex. What happens depends on the situation, on what you’ve been through before, on timing and on the state your body is in. The popular story of stress as “primitive brain versus rational brain” is appealing, but the real brain is messier. And the messier version is more useful.

In plain words

Stress does odd things to memory. You might forget what someone said five minutes ago, and still remember an embarrassing moment from years ago in sharp detail.

Memory works in steps, and stress hormones affect each one differently. Encoding is taking something in at the time. Consolidation is the memory settling into something lasting over the next hours and days, and a lot of that happens while you sleep. Retrieval is getting it back out later.

One of the better-established findings is that sudden stress can make it harder to get back things you already know. At the same time, stress hormones can help emotional moments settle in more firmly. Your brain is treating the moment as important, and its message is closer to “this mattered, pay attention” than “remember every detail.”

That can be useful. If something dangerous happens, remembering the warning signs can help you avoid it next time. Your brain isn’t a perfect judge of what to keep, though, which is how one humiliating sentence from a Tuesday in 2017 can stay with you for years. Brains are efficient, but they aren’t very tidy!

When you’re stressed, your attention tends to lean toward anything that seems connected to the problem. That’s handy when there’s real danger in front of you. It helps far less when the “danger” is an unanswered message, or something that can’t be settled tonight.

And it can turn into a loop. The more you notice the problem, the more important it feels, and the more you notice things connected to it. Everything else starts to feel far away. That’s why stress can wear you out even when you’re not doing much. Your brain is working hard, and nobody can see that work from the outside.

Then there’s the hippocampus, a part of the brain that’s important for memory and for making sense of context. It’s also involved in keeping the stress response in check.

Context is what tells you the difference between “this is dangerous” and “this reminds me of something dangerous.” A loud bang means one thing when you’re walking alone somewhere unfamiliar at night, and something else when you’re safe at home and it turns out to be the heating. Context changes what something means.

You may have read that stress “shrinks your hippocampus.” That’s an oversimplification: most of the evidence comes from animal studies, and a difference on a brain scan can’t show that stress caused a change in one particular person.

The fairer summary is that long-term stress can affect memory and emotion, for better or worse, briefly or lastingly.

In plain words

Your body is good at adapting. All day long it keeps adjusting to what’s happening, so your heart rate, blood sugar, hormones, attention and immune system all shift. Researchers call this allostasis, which roughly means staying stable by changing.

Adapting has a cost, though. If your stress systems switch on too often, stay on too long or don’t switch off properly, wear and tear builds up. Researchers call this allostatic load. Think of it as “my body keeps being asked to work hard,” more than “my cortisol is always high.”

Allostatic load isn’t a diagnosis, and no simple blood test can tell you how stressed you are. Researchers use it to study several body systems at once, and it’s complicated to measure. Your nervous system is not a phone battery sitting at 17%. Your body is a lot more interesting than that!

It helps to tell short-term stress from long-term stress. Short-term stress, also called acute stress, comes and goes. You deal with the situation, and ideally your body settles back down. Long-term stress, also called chronic stress, is different, because the pressure keeps going, keeps coming back or feels impossible to escape. That might be money worries, caring for someone, a long-term illness, or lots of small stresses that never give you a chance to settle.

Being able to react to stress is healthy. The problem is not getting the chance to recover. So managing stress shouldn’t only mean learning to put up with it forever. Recovery is part of the biology!

In plain words

Stress and sleep have an irritatingly two-way relationship. Stress can get in the way of sleep, and poor sleep can make your stress system react more strongly.

So you can end up in a loop that keeps itself going. That’s how someone can say, completely truthfully, “I’m exhausted, but I can’t switch off” (also known as 2 a.m., phone in hand).

Stress doesn’t stop at the brain. Your nerves, hormones and immune system are in constant conversation, so stress that keeps going can change how inflammation is controlled and add wear and tear to the body.

That doesn’t mean stress just “causes inflammation,” or that every symptom is cortisol’s fault. The links between stress, inflammation and illness vary from person to person and from one condition to another.

The basic idea is still solid enough to take seriously. What you go through in your mind can change how your body works, and your brain is one of the main coordinators of that. You’re not a brain floating around inside a body. Your brain is part of your body, and stress reminds us of that pretty forcefully.

In plain words

Some people seem to handle stress better than others, and it’s easy to think they were born with a better nervous system. It’s much less fixed than that. People differ in how strongly they react, how quickly they recover, what their past has taught them to expect and how much support they have.

Researchers increasingly see resilience, the ability to cope and recover, as something that changes with your circumstances. The same person can handle a sudden crisis well and struggle with months of uncertainty. There’s no single “resilient brain.” How you cope is shaped by biology, experience, sleep, relationships, your surroundings, what you do, and probably plenty of things nobody understands well yet.

A life with no stress at all wouldn’t be good for you, either. You need to be able to get alert when something matters, find energy when you need it, feel excited and care how things turn out. The goal is flexibility. Can your system switch on when something matters, and stand down when it’s over? Can your attention come back to other things? Can your brain tell a problem happening now from an old one? Can your body feel safe without looking straight away for the next threat?

That’s also why “just relax” isn’t much help. Sometimes you can’t think your way out of a state your body is in. Your brain and body affect each other in both directions, though, so what you do, think and notice can change how your body settles. This is why things that look almost insultingly ordinary can matter!