Cortisol Circadian Rhythm

Cortisol is the body's principal glucocorticoid, a steroid hormone made by the zona fasciculata of the adrenal cortex. It mobilizes glucose, modulates immunity, raises blood pressure, and sharpens the brain — but its single most striking feature is that it is never secreted at a constant level. Instead it follows a deep, reliable circadian rhythm, layered with faster pulses, that anchors the body's daily transition from sleep to wakefulness. Understanding this rhythm is the key to interpreting almost every clinical cortisol measurement, because the same value can be normal or pathological depending entirely on the time of day it was drawn. (This article is educational and is not medical advice.)

How the rhythm is generated

The timing comes from the suprachiasmatic nucleus (SCN), a cluster of about 20,000 neurons above the optic chiasm that serves as the body's master clock. Light hitting melanopsin-containing retinal ganglion cells resets the SCN each morning, keeping its near-24-hour molecular oscillator entrained to the solar day. The SCN does not make cortisol; it dictates when cortisol is made.

The output machinery is the HPA axis, a three-tier cascade:

• Hypothalamus. Parvocellular neurons of the paraventricular nucleus release corticotropin-releasing hormone (CRH) and arginine vasopressin into the hypophyseal portal blood.
• Anterior pituitary. CRH drives corticotroph cells to release adrenocorticotropic hormone (ACTH), cleaved from the precursor pro-opiomelanocortin.
• Adrenal cortex. ACTH binds the melanocortin-2 receptor on zona fasciculata cells, stimulating cholesterol delivery via StAR protein and the synthesis of cortisol within minutes.

The SCN biases this whole cascade so that CRH and ACTH output rises in the late night and early morning. It also sends a direct neural signal through the autonomic nervous system to the adrenal gland, making the cortex transiently more sensitive to a given pulse of ACTH around waking. The result is the familiar curve: a nocturnal rise, a sharp early-morning peak, a daylong decline, and a midnight trough.

The numbers that define normal

In a conventionally sleeping adult, plasma cortisol begins climbing around 02:00-03:00. It reaches its diurnal peak of roughly 10-20 µg/dL (275-550 nmol/L) between 06:00 and 08:00, then declines through the day to less than 5 µg/dL (under ~140 nmol/L) by late evening, with the true nadir near midnight. Total daily secretion is about 10-20 mg of cortisol — far less than the 100+ mg per day textbooks once cited before isotope-dilution studies corrected the figure. Cortisol circulates ~90% bound to corticosteroid-binding globulin (transcortin) and albumin; only the free fraction is biologically active, which is why salivary cortisol (free hormone) tracks tissue exposure better than total serum.

Two faster rhythms ride on top of the circadian wave. The first is ultradian pulsatility: roughly 15-18 discrete secretory bursts per 24 hours, each lasting tens of minutes, generated by the feed-forward/feed-back loop between pituitary ACTH and adrenal cortisol. The second is the cortisol awakening response (CAR) — an additional 50-75% surge in the first 30-45 minutes after waking, triggered by the act of awakening itself rather than by the clock alone. Glucocorticoid receptors read the pulsatile pattern, not just the average, so the shape of the signal carries information.

Negative feedback and the dexamethasone test

Cortisol restrains its own production. It binds glucocorticoid receptors in the hypothalamus and pituitary, suppressing CRH and ACTH — classic negative feedback that prevents runaway secretion. Clinicians exploit this loop directly. In the overnight 1 mg dexamethasone suppression test, a synthetic glucocorticoid taken at 23:00 should suppress the next morning's cortisol below ~1.8 µg/dL in a healthy axis. Failure to suppress signals autonomous cortisol production, the hallmark of Cushing syndrome. The mirror-image abnormality — a low morning cortisol with an inappropriately high ACTH — points to primary adrenal failure, because the pituitary is shouting at an adrenal gland that cannot answer.

Clinical correlations: when the rhythm breaks

Because the rhythm is so stereotyped, its disruption is diagnostically powerful:

• Cushing syndrome. Chronic cortisol excess (from a pituitary adenoma, adrenal tumor, or ectopic ACTH) first erases the midnight nadir. A late-night salivary cortisol that stays high is among the most sensitive screening tests, often abnormal before total daily output rises.
• Adrenal insufficiency. In Addison disease the adrenal cortex is destroyed, so morning cortisol is low and fails to rise; ACTH climbs and its melanocortin activity darkens the skin. Secondary insufficiency from pituitary disease or — far more commonly — from suppression by exogenous steroids gives low cortisol and low ACTH.
• Steroid withdrawal. Long-term synthetic glucocorticoids suppress CRH and ACTH; the HPA axis can take weeks to months to recover, which is why steroids are tapered, not stopped abruptly. Adrenal crisis during stress is the feared consequence.
• Circadian misalignment. Rotating shift work and jet lag uncouple the cortisol peak from the wake time. The SCN re-entrains only ~1 hour per day, so workers are flooded with cortisol at the wrong time and run dry when they need alertness — a pattern linked to metabolic syndrome and cardiovascular risk.
• Depression and chronic stress. Major depression is classically associated with HPA-axis overactivity and a flattened, non-suppressing rhythm; chronic stress and burnout more often blunt the awakening response.

Cushing syndrome vs Addison disease

The two ends of the cortisol spectrum are easiest to grasp side by side. Both distort the circadian curve, but in opposite directions, and the ACTH level tells you where the lesion sits.

Feature	Cushing syndrome (cortisol excess)	Addison disease (primary cortisol deficiency)
Cortisol level	High; loss of midnight nadir	Low; no morning rise
Circadian rhythm	Flattened — high even at night	Flattened — low throughout
ACTH	High (pituitary/ectopic) or low (adrenal tumor)	High (adrenal cannot respond)
Dexamethasone suppression	Fails to suppress	Not the relevant test
Key screen	Late-night salivary cortisol; 24-h urinary free cortisol	08:00 cortisol; ACTH stimulation (cosyntropin) test
Classic signs	Central obesity, striae, hypertension, hyperglycemia	Fatigue, hypotension, hyperpigmentation, hyponatremia

What the morning peak actually does

The pre-waking cortisol surge is not arbitrary. It prepares the body to face the day: it stimulates hepatic gluconeogenesis to top up blood glucose after the overnight fast, mobilizes free fatty acids and amino acids, sensitizes the vasculature to catecholamines to support blood pressure on standing, and modulates the immune system's daily rhythm of cytokine release. It also influences mood and cognition, partly explaining why early-morning hours can feel either alert or anxious depending on the size of the surge. Because cortisol is permissive for so many systems, the morning peak effectively synchronizes peripheral clocks in the liver, fat, and muscle to the central SCN clock, making cortisol a key messenger that keeps the whole body on the same schedule.

Common misconceptions

• "A single cortisol level tells you if it's normal." Without the draw time it is nearly meaningless — a value normal at 08:00 may be grossly abnormal at midnight.
• "Cortisol is just the stress hormone." Stress reactivity is layered on top of a baseline circadian rhythm that runs every day, stress or not.
• "The adrenal gland sets the rhythm." The timing comes from the SCN and HPA axis; the adrenal is the effector that follows ACTH and direct neural cues.
• "More cortisol is always worse." The morning peak is essential; it is the loss of rhythm — high troughs or absent peaks — that signals disease.
• "The awakening response is just the morning rise." The CAR is a separate, awakening-triggered surge superimposed on the circadian curve.