Cortisol and the Stress Response: What It Does and What Is Myth

What is the stress response?

When the body faces a threat or challenge, two systems answer.

• The fast system. The sympathetic branch of the autonomic nervous system releases noradrenaline and, from the adrenal glands, adrenaline. Heart rate, breathing and blood pressure change within seconds.
• The slow system. The brain's stress centres release corticotropin-releasing hormone, which leads the pituitary and then the adrenal glands to release cortisol. This chain is called the HPA axis (hypothalamus, pituitary, adrenal). Its effects build over minutes to hours.

Together they form a stress system that reaches metabolism, immunity, sleep, mood and reproduction [S1].

What does cortisol do?

Cortisol is often called "the stress hormone", but it is an essential hormone that the body needs every day. It helps release energy from stores, shapes the immune response and helps set daily rhythms. Both too much and too little activity of the stress system lead to disease [S1]: without cortisol, the body cannot cope with illness or injury.

Its daily rhythm

Cortisol follows a strong daily rhythm. It rises sharply in the half hour or so after waking, a pattern called the cortisol awakening response, and measuring it reliably depends on taking samples at exact times after waking [S2]. Levels fall through the day and reach a quiet period in the evening and early night [S3].

This is why a single cortisol value means little on its own. A high morning reading can be normal, and the same reading in the late evening could be a sign worth checking. Sleep matters too: in a laboratory study, losing sleep raised cortisol the following evening and delayed its evening drop [S3].

How is cortisol measured?

• Blood: a snapshot at one moment; used in clinical testing.
• Saliva: reflects free cortisol and can be collected at home at set times, for example to study the awakening response or in late-night testing [S2] [S5].
• Urine: collected over a full day, it gives a daily total; used in clinical testing [S5].
• Hair: reflects cortisol over months and is mostly a research method [S4].

Each method answers a different question, and none of them is done by a watch, ring or phone. Wearables measure heart signals such as heart rate variability, not hormones.

What changes cortisol?

• Sleep loss raises evening cortisol [S3].
• Exercise raises cortisol at moderate to high intensity, but not at low intensity, in a small study [S8].
• Acute stress activates the HPA axis as part of the normal response [S1].
• Time of day matters more than most single influences [S2].

Practices. Context-dependent and mixed. Meta-analyses of meditation report lower cortisol in some analyses [S10], but one found the effect only in people with an illness, and a small, non-significant effect in saliva samples [S9]; see meditation and the nervous system. In a small randomised trial, weeks of diaphragmatic breathing training lowered salivary cortisol [S11]; see slow breathing. Cold water immersion did not raise cortisol in a small laboratory study [S12]; see cold exposure.

Cortisol in medicine

Real cortisol disorders exist and are diagnosed with specific tests:

• Cushing's syndrome (too much cortisol) is tested with urine cortisol, late-night saliva cortisol or a dexamethasone suppression test, confirmed with a second test and an endocrinologist [S5].
• Adrenal insufficiency (too little cortisol) is tested in people with suggestive symptoms, using a corticotropin stimulation test [S6]. It can be life-threatening.

See a doctor if you have unexplained weight gain with easy bruising or new high blood pressure, or persistent exhaustion with weight loss, dizziness on standing, nausea or salt craving. For questions to bring, see an ONDA report for your endocrinologist.

Is "adrenal fatigue" real?

"Adrenal fatigue" is often described as adrenals worn out by chronic stress. A systematic review found no substantiation that it is an actual medical condition [S7], and endocrinology societies do not recognise it. Exhaustion is real and worth investigating, but tests and supplements sold for "adrenal fatigue" can delay finding the actual cause, including true adrenal insufficiency [S6].

Can a practice or supplement "lower cortisol by X%" or "detox" it?

Claims that a breathing exercise, food or supplement lowers cortisol by a specific percentage rarely come with a study, and the research on practices shows mixed, context-dependent effects [S9] [S10]. Cortisol is not a toxin to be removed: it is a hormone the body needs every day [S1]. A real excess of cortisol is a medical condition diagnosed with specific tests, not something a "cortisol detox" fixes [S5].

Can your watch or HRV show your cortisol?

No wearable measures cortisol. Watches and rings estimate heart signals such as heart rate and HRV, which can shift with stress and sleep, but they are not a cortisol reading. A single low HRV reading does not by itself mean you are stressed or unwell. Cortisol is measured in blood, saliva, urine or hair [S4] [S5].

What it does not tell you

• A single cortisol number says little without the time it was taken [S2].
• A practice that lowers cortisol in one study may not do so for you, or in saliva rather than blood [S9].
• Feeling stressed is not the same as an abnormal cortisol level.

In ONDA

ONDA does not measure cortisol. ONDA shows your pulse live during a practice, from the iPhone camera or an Apple Watch. Changes in your HRV or resting heart rate can reflect sleep, stress, illness or training, but they are not a cortisol reading; see why HRV changes day to day.