Cortisol: the messenger we started punishing

The biggest hormone peak of your day happens before you have done anything. And nobody knows exactly what it is for.

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About half an hour after you opened your eyes this morning, a substance in your blood reached its highest level of the entire day.

You did nothing to bring it about. You were still lying down.

That peak is so reliable that researchers use it to check whether participants collected their saliva samples at the right moment. Thirty to forty-five minutes after waking, every single morning.¹

And this is where it gets strange. Why would your body produce its largest hormone peak of the day, every morning, with that kind of precision?

The honest answer is that we do not know for certain.

The common idea is that the body is getting ready for whatever the day will ask of it. Researchers call this the anticipation hypothesis, and that name says enough. It is a hypothesis. The precise function of the largest cortisol rise in your day has still not been clarified.²

Sit with that for a moment.

The hormone half the internet is warning you about is the same hormone whose most important peak we do not yet fully understand. And still, every video knows exactly how you should lower it.

Perhaps you are reading this because you are tired. Not tired from one bad night, but that other kind of tired, where you already start the morning behind. And then you come across an explanation online that finally makes sense of it: your cortisol is too high.

That relief is real. At last there is a name.

In this piece I am going to take that name away from you. And I will give you something back that is worth more.

What this hormone actually does for you

Cortisol is made in the adrenal cortex, the outer layer of two small glands that sit like caps on top of your kidneys. Together they weigh less than ten grams.

They decide nothing on their own. There is a chain of three stations.

Deep in your brain sits the hypothalamus, a small region that watches over your inner world: temperature, hunger, thirst, sleep. When it notices that something is being asked of you, it sends a signalling substance to the pituitary, the gland just below it. The pituitary passes the instruction into your blood. That signal reaches your kidneys, and there the cortisol is made.

Request, instruction, delivery. Doctors call this chain the HPA axis.

The cleverest part comes next. Once there is enough cortisol, cortisol itself turns the first two stations down again. The system has its own brake built in.³

And what does it do for you? It makes fuel available in your blood. It helps keep your blood pressure up. It limits inflammatory responses so they do not run on endlessly. It influences your alertness and your memory.³

That sounds abstract, until you look at the people who lack it.

There is a condition in which the adrenal glands make too little cortisol: adrenal insufficiency. These people take hydrocortisone every day, a medicine that replaces the missing hormone. But the most telling part comes later in the guideline. When they are ill or feverish, they must increase the dose. They carry a steroid card and keep an emergency injection within reach.⁴

Read that again. There is a group of people for whom ordinary flu becomes dangerous without extra of this hormone. With a syringe in the drawer in case things go wrong.

Then “the lower the better” cannot be biologically true. Not slightly untrue.

That we know any of this at all is recent. Only in 1936 did the researcher Hans Selye describe that very different harmful stimuli produced the same kind of response in laboratory animals, without knowing which hormone lay behind it.⁵

Which leaves the question of why this hormone is allowed to run so high in the morning, and not in the evening.

Why the morning is allowed and the evening is not

Your cortisol starts rising in the final hours of your sleep, so before you are awake. Around waking it is high, and half an hour later it reaches the peak of the day. After that it falls, all day long, until it almost falls silent in the evening and the first part of the night. Within that daily rhythm it is also released in pulses, in waves roughly an hour apart.³

There is a practical lesson in that straight away. One cortisol reading is a single still from a film that moves all day. A result that is perfectly normal at eight in the morning is cause for concern at eleven at night. Without a time of day, the word “high” means nothing.

Then the question you may already have had. Does cortisol make way for melatonin in the evening? Do the two hand over to each other?

No. And it is a lovely misunderstanding to put right.

Deep in your brain lies a cluster of cells the size of a grain of rice, the suprachiasmatic nucleus. That is your central clock: about ten thousand nerve cells on each side, and they direct the daily cycles of your behaviour and your body.⁶

Cortisol and melatonin do not pass a baton to one another. They are two hands on the same clock. One points to the morning, the other to the night, and both are moved by the same mechanism.

That difference is not only theory. It means that with light, sleep and timing you are not acting on one hormone, but on the clock that drives them both. Remember that, because by the end of this piece it is the only thing you genuinely have in your hands.

The clock decides when your cortisol rises. But what decides how high?

What your cortisol does and does not respond to

Three things are undisputed, and they are more concrete than you would think.

Waking up. You know that one now.

Physical exertion. Twelve trained men did thirty minutes of exercise at three intensities. At light intensity their cortisol barely moved, around 6%. At moderate intensity it went to roughly 40%. At high intensity, to more than 83%.⁷

More than 83%, in people doing something we all call healthy. Had you tested that man after his session, a home kit could have talked him into a cortisol problem.

Illness, fever and pain. That is precisely why people without cortisol of their own have to take more of it when they catch flu.⁴

Which leaves the psychological side. That is where the biggest misconception of all sits.

In 1993 German researchers described a test that is now used all over the world, named after the city of Trier. The set-up is cruelly simple. You get ten minutes to prepare, and then you must give a job-interview speech and do mental arithmetic in front of an audience that watches you in silence. Cortisol in the saliva rose reliably to two to four times the starting level.⁸

But why does this work so well, and a deadline or a house move not?

Two American researchers laid 208 laboratory studies side by side to find out. An unpleasant task on its own did little. What did produce a clear response were tasks with two specific features: you could do nothing about them, or you were being judged by others. When both were present, the rise was largest and recovery took longest. The researchers write it plainly in their own summary: this contradicts the belief that cortisol responds to all types of stressors.⁹

Read those two words again. Uncontrollable. Judged.

That is not a coincidental pairing. For a group animal, being excluded was once life-threatening, and helplessness more dangerous still.

And it explains something you have probably lived through without having words for it. Why one busy year broke you, and another year, with just as much work, did not. The difference rarely lay in the amount. It lay in whether you had any say, and whether someone was watching who passed judgement on you.

If all of this is normal, when does it become a disease?

When is it truly too much?

That line exists, and it is drawn more sharply than the hype suggests.

Cushing’s syndrome results from lengthy and inappropriate exposure to excessive glucocorticoids.¹⁰ There are two roads to it: long-term use of medicines such as prednisolone, or the body itself, usually through a small benign tumour. It is rare, and tellingly the very first step in the diagnostic guideline is not a test but a question: is this person taking those medicines themselves?¹¹ Once it is established, treatment is essential, and surgery comes first.¹²

That is the whole disease, in one paragraph. The chance that your tiredness comes from here is small, and that is exactly the point.

So what happens with ordinary, long-term stress?

Three researchers gave their 2007 review a title that says it all: if it goes up, must it come down? They gathered the research on long-term stress and the HPA axis, and found that the results had been contradicting each other for years.¹³

When they untangled that contradiction, it turned out not to be noise. It was information.

Timing is decisive. At the onset of a long-term burden the activity is raised, and as time passes it falls. The type of burden matters too: situations that threaten you physically, that are traumatic, or that you have no influence over produce a high, flat daily profile. And in people with post-traumatic stress disorder the activity was found to be lower.

Chronic stress therefore does not automatically mean high cortisol. It can be high, it can be flattened, and it can be low.

Which pulls the floor out from under the most popular sentence of the moment. “Your cortisol is too high” is not a conclusion anyone can draw from your symptoms.

And precisely here the phrase appears that you have surely heard: exhausted adrenals, or adrenal fatigue. Two Brazilian endocrinologists tested that idea systematically in 2016. They kept 58 usable studies and found no substantiation whatsoever for the existence of the condition. Their conclusion is literally in the title: adrenal fatigue does not exist.¹⁴ A review from within endocrinology now counts it among the pseudo-endocrine disorders: proposed conditions that have never been proven, diagnosed with non-validated questionnaires and salivary profiles, and treated with remedies that are sometimes dangerous.¹⁵

I want to be careful here, because there is a person behind this sentence.

That the diagnosis does not exist does not mean your tiredness does not. The sleepless nights are real. The inability to get going in the morning is real.

But a false label is not free. It costs you the real investigation. Of your thyroid, your blood count, your iron, your sleep, your medication, your mood, your life. Whoever sells you a cortisol problem also sells you the months in which nobody looked at the rest.

That such a system is not free of charge is true, by the way. The American neuroendocrinologist Bruce McEwen gave that a name in 1998: allostatic load, the bill for adapting over and over again.¹⁶ It is only that this bill cannot be read off a single number.

If your hormone does not tell you what is wrong, what does? That answer does not lie in your blood. It lies in your evening.

Your evening, your light and your sleep

High cortisol in the morning is part of the design. High cortisol in the evening is something else, because by then it should be almost silent.

Researchers brought together 80 studies, yielding 179 associations, and looked at the slope of the daily curve: the difference between morning and evening. A flatter curve was associated with poorer health, most strongly for immunity and inflammation.¹⁷

And here the nuance belongs immediately, because it is considerable. The average association was small. Across large groups it is visible, but for one individual it says almost nothing. An association is not a diagnosis.

So what flattens that evening? Two things are well researched, and both say something no test can tell you.

Sleeping badly. Researchers followed young, healthy men across some thirty hours. After a night with little sleep, their cortisol the following evening was about 37% higher than normal. After a night with no sleep at all it was 45%. And the quiet phase, the moment when production normally falls silent, began at least an hour later.¹⁸

Notice the order, because this whole article sits inside it. The bad night came first. The hormone came after.

Light in the evening. In a tightly controlled study, participants read before bed either on a light-emitting screen or from a printed book. After the screen they had less melatonin, their biological clock shifted later, they took longer to fall asleep and they were less alert the next morning.¹⁹

Be precise here, because this is where almost everyone moves too fast. In that study melatonin was measured, not cortisol. It does not say that your phone raises your cortisol.

What it does say is subtler. Light in the evening shifts the clock that drives cortisol and melatonin alike. So you are not shifting one hormone. You are shifting the time your body believes it is living in.

And that can be changed. In 2025, researchers had 111 healthy students reduce their screen time to a maximum of two hours a day for three weeks. On stress, sleep quality, well-being and low mood they saw small to medium improvements.²⁰ Here too, no more should be made of it than it is: young, healthy students, a small group, and no cortisol measured. Afterwards the screen time simply shot back up.

What you are left with is not a trick. It is, however, the only part you can genuinely reach. A fixed time to go to bed. Less light in the final hours. Movement during the day, where the rise belongs. And for persistent symptoms, a doctor who first rules out the ordinary causes.

This is useful, and it costs nothing. Which is precisely why you hear it almost nowhere.

Why cortisol is suddenly everywhere

Because this is not a new discovery, but an old business model with a new algorithm.

On 5 October 2004 the American trade and consumer protection regulator took action against the marketers of two products: CortiSlim and CortiStress. The advertising claimed that controlling cortisol was “the answer” to weight loss. Users would lose four to ten pounds a week, week after week. The infomercials were designed to look like episodes of a talk show called Breakthroughs.²¹

Twenty-two years ago. The same promise, a different screen.

How medical tests are sold today was mapped out in 2025. Researchers analysed 982 posts on Instagram and TikTok about five popular tests, from accounts with more than 194 million followers between them. Benefits were mentioned in 87% of the posts, harms in 15%. More than 83% had a promotional tone. Scientific evidence was explicitly used in 6% of posts. In 68% of the accounts there was a financial interest.²² This research was not about cortisol but about five other tests, so it says something about how tests are sold, not about cortisol in particular.

Why does cortisol lend itself to this so well? It is real, it is measurable, and the symptoms attached to it are so general that almost everyone recognises themselves in them.

That is the trick. Marketing takes ordinary, often painful human experiences and turns them into one recognisable hormonal diagnosis. Not because one hormone can explain all those symptoms, but because one explanation is easier to sell than the complicated truth.

And notice how airtight the model is. If your cortisol is high, that is called overload. If it is low, that is called exhaustion. If the pattern is odd, that is called dysregulation. There is no result that proves the seller wrong.

Does this mean rest and meditation are nonsense? No. But their value should not depend on the promise that one hormone reading will fall. A systematic review of six studies on mindfulness and cortisol found changes in studies without a control group, and precisely not in the randomised ones.²³ Meditation can give you a great deal. It does not need a laboratory to prove it.

Twenty-five centuries ago, Sophocles had his guard, the man who had to bring the bad news, say exactly what we now do to this hormone.

For no man delights in the bearer of bad news.

Sophocles, Antigone

From the classic English translation by R. C. Jebb.²⁴

At the start of this piece I took a name away from you. This is what I give back.

You are not broken. You have a system that switches on for you every morning, whose most important peak we still do not fully understand, and that responds most strongly to precisely the two things a human being finds hardest: having no grip, and being watched.

That is not a diagnosis. It is a direction.

Health is not the lowest possible cortisol. Health is a system that can fire up when it is needed, do its work, and fall silent again afterwards.

Where in your life do you currently have the least say? And when did you last take something back, however small?

This article provides general information and is not medical advice. If you recognise yourself in persistent tiredness, poor sleep or other symptoms, please speak with your GP or a treating specialist. If you take glucocorticoids such as prednisolone, never stop or change them on your own initiative.

Related reading

• Fear and control: the smoke detector in your head
• The unconscious brain: your first reaction is a suggestion
• Why sleeping in gives you a headache

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Sources on this site are cited in APA style, with a DOI wherever one exists. Why I made that choice →