Cortisol Evening Test

Your body runs on a 24-hour hormonal clock, and cortisol is one of its most tightly regulated hands. This stress hormone should peak in the early morning to help you wake up, then fall steadily throughout the day, reaching its lowest point around midnight. When that nighttime dip disappears, it is one of the earliest and most reliable signs that something has gone wrong with your adrenal system.

Evening cortisol testing catches this specific failure. A standard morning cortisol draw cannot detect the problem, because cortisol is supposed to be high in the morning in both healthy and unhealthy people. The diagnostic power lies in checking when levels should be at their floor. If they are not, it signals a disruption in the feedback loop between your brain and your adrenal glands, a system called the HPA (hypothalamic-pituitary-adrenal) axis.

What Cortisol Does and Why Timing Matters

Cortisol (full name: cortisol, sometimes called hydrocortisone) is a steroid hormone made from cholesterol in a region of the adrenal glands called the zona fasciculata. Only about 5 to 10% of cortisol in your blood circulates freely. The rest rides on carrier proteins. That free fraction is the biologically active form, the one that enters cells and affects gene activity controlling blood sugar, immune function, fat storage, and blood pressure.

The rhythm matters as much as the amount. Your brain's master clock, a tiny structure called the suprachiasmatic nucleus, coordinates cortisol release so it surges before waking and bottoms out during early sleep. This pattern is not just a quirk of biology. Losing the nighttime low point is the hallmark biochemical abnormality in Cushing syndrome, a condition of chronic cortisol excess that damages nearly every organ system.

Heart Disease and Cardiovascular Mortality

Elevated evening cortisol is not just a marker of endocrine disease. It independently predicts cardiovascular death, even in people without a diagnosis of Cushing syndrome. The connection has been demonstrated across multiple large, long-running studies.

In the Whitehall II Study, which followed over 4,000 adults for an average of 6.1 years, people whose cortisol failed to drop normally across the day (a flatter slope from morning to night) were nearly twice as likely to die from cardiovascular causes (about 1.9 times the risk per standard deviation flattening in slope). That association held after adjusting for age, sex, smoking, alcohol, physical activity, BMI, diabetes, hypertension, and cholesterol.

The German KORA-F3 Study tracked about 1,090 people for 11 years and found that each standard deviation increase in late-night salivary cortisol raised cardiovascular mortality risk by roughly 49%. A strong drop from peak to bedtime cortisol was protective, cutting cardiovascular death risk in half. Elevated evening cortisol was not linked to non-cardiovascular deaths, suggesting a specific effect on the heart and blood vessels rather than a general marker of poor health.

Among hypertensive adults, the pattern repeated. In a cohort of over 2,300 people followed for a median of 7.2 years, higher midnight cortisol was associated with about 24% greater risk of cardiovascular events per standard deviation increase.

All-Cause Mortality

The Italian InCHIANTI Study of 861 adults aged 65 and older found that those in the highest third of 24-hour urinary cortisol were five times more likely to die from cardiovascular causes over 5.7 years, after adjusting for existing heart disease and standard risk factors. The Longitudinal Aging Study Amsterdam, following nearly 1,000 older adults for up to 7.5 years, found that women with high evening cortisol had about 1.8 times the mortality risk of those with low evening cortisol.

Sources: Whitehall II Study (Kumari et al.), KORA-F3 Study (Karl et al.), InCHIANTI Study (Vogelzangs et al.)

What this means for you: a persistently elevated evening cortisol is not a benign finding. Even outside the context of Cushing syndrome, a flattened cortisol rhythm carries real cardiovascular consequences. If your evening cortisol is elevated, following up with cardiovascular risk assessment and retesting to confirm the pattern is a reasonable next step.

Cushing Syndrome: The Primary Clinical Indication

Evening cortisol testing was originally developed to diagnose Cushing syndrome, a condition where the body produces too much cortisol for too long. The disease is uncommon (roughly 2 to 8 new cases per million people each year), but it is frequently missed for years because its symptoms overlap with far more common conditions: weight gain, high blood pressure, diabetes, depression, and thinning bones.

The most common cause is a small, benign tumor in the pituitary gland (accounting for 65 to 70% of cases) that overproduces ACTH, the signaling hormone that tells the adrenals to make cortisol. Other causes include tumors elsewhere that secrete ACTH, adrenal tumors that produce cortisol on their own, and, most commonly of all, long-term use of prescription corticosteroid medications like prednisone.

Left untreated, Cushing syndrome carries 2 to 5 times the normal mortality risk. Its metabolic effects include hyperglycemia, central obesity, osteoporosis, muscle weakness, immunosuppression, and mood disorders. The cardiovascular toll is especially severe.

Cancer Outcomes

A disrupted cortisol rhythm also appears to worsen cancer prognosis. In a study of 113 women with ovarian cancer, those with elevated nighttime cortisol survived an estimated 3.3 years compared to 7.3 years for those with low nighttime cortisol. Each standard deviation increase in night cortisol was associated with a 46% greater likelihood of death. In 62 lung cancer patients, a flatter daytime cortisol slope predicted earlier death and was linked to lower immune cell counts.

These findings are observational and do not prove that cortisol itself causes worse cancer outcomes. But they suggest that the integrity of your cortisol rhythm may reflect broader immune and stress-response capacity that matters during serious illness.

Adrenal Incidentalomas: A Growing Use Case

Adrenal incidentalomas are benign-appearing growths on the adrenal glands discovered by accident during imaging for unrelated reasons. They are common: found in 1.4 to 7% of adults, and up to 10% of people over 70. Between 20 and 50% of these produce a small amount of extra cortisol, a condition called mild autonomous cortisol secretion (MACS), without triggering the full picture of Cushing syndrome.

MACS is not harmless. An international study across 30 centers in 16 countries followed 3,656 patients with adrenal incidentalomas for a median of 7 years. Those with autonomous cortisol secretion had about 1.8 times the mortality risk compared to those with non-functioning growths, after adjusting for age, sex, and other health conditions. A separate Swedish cohort of over 1,000 patients showed a dose-response pattern: more cortisol secretion meant higher mortality. Current guidelines recommend that everyone with an adrenal incidentaloma be screened for cortisol overproduction.

Reference Ranges

Evening cortisol thresholds vary substantially depending on which lab method is used, which is why comparing your results within the same lab over time matters more than fixating on a single number. The most commonly tested form is late-night salivary cortisol (LNSC), collected at home around 11 PM to midnight.

These thresholds were developed primarily to diagnose Cushing syndrome, not to define an "optimal" range for longevity or general health. No published research has established target evening cortisol levels for preventive health optimization. What the evidence does support is that a healthy cortisol rhythm should show a clear, deep nighttime trough. The absence of that trough is the red flag.

Age also shifts the goalposts. Evening cortisol levels increase 20 to 50% between ages 20 and 80. The nighttime low point progressively rises, and the overall daily swing shrinks. People over 50 have significantly higher late-night salivary cortisol than younger adults (median 5.24 nmol/L vs. 3.31 nmol/L in one study). This means an elevated reading in an older adult needs to be interpreted more carefully, and ideally compared against age-matched data.

Why One Reading Is Not Enough

Cortisol is one of the most variable hormones in your body. The day-to-day coefficient of variation (how much a measurement bounces around in the same person) is roughly 18 to 27%. Up to half of patients show meaningful variability between consecutive late-night salivary cortisol samples. This is why every major guideline recommends at least two, and preferably three, measurements before drawing conclusions.

This variability is especially relevant if you are tracking a trend rather than diagnosing Cushing syndrome. A single elevated reading could reflect a stressful day, a late meal, poor sleep the night before, or nothing at all. Two or three readings collected on separate nights, following the same bedtime routine, give you a far more reliable picture of your true cortisol rhythm.

For anyone making lifestyle changes intended to improve their cortisol pattern, such as improving sleep habits, managing stress, or adjusting exercise timing, retesting after 3 to 6 months provides the best signal of whether the intervention is working. After that, annual monitoring is reasonable to catch any drift. If results are borderline or inconsistent, consider collecting samples on three separate nights within the same week to average out the noise.

When Results Can Be Misleading

Several common situations can distort an evening cortisol reading enough to lead you to the wrong conclusion. Knowing these pitfalls helps you avoid unnecessary alarm or false reassurance.

• Shift work or jet lag: the cortisol trough is tied to your sleep schedule, not the clock. If you work nights or recently crossed time zones, your low point may fall at a different hour, making a standard 11 PM sample meaningless. Wait until you have been on a stable sleep-wake cycle for at least a week before testing.
• Recent acute illness or surgery: cortisol can spike up to six-fold during acute stress and may take 24 to 48 hours (or longer after severe illness) to normalize. Testing during or immediately after illness will give a falsely elevated result.
• Medications: prescription corticosteroids (prednisone, dexamethasone, inhaled steroids, topical creams) suppress your natural cortisol production and can make results uninterpretable. Oral estrogens (including birth control pills) raise the carrier protein for cortisol, inflating total cortisol numbers without changing the biologically active free fraction. Biotin supplements can interfere with certain assay methods.
• Late-night eating: a meal close to bedtime can raise cortisol. One study found that eating at 10 PM increased cortisol awakening response the next morning compared to not eating. Collect your sample at least two hours after your last meal.

Diagnostic Power Compared to Other Tests

For detecting Cushing syndrome, late-night salivary cortisol is one of three recommended first-line screening tests. It is the most patient-friendly, collected at home without fasting or medication. Its sensitivity (92 to 100%) and specificity (85 to 100%) are generally superior to both the 24-hour urinary free cortisol collection (sensitivity 45 to 98%, specificity 45 to 98%) and the 1 mg dexamethasone suppression test (sensitivity 80 to 95%, specificity 80 to 95%).

Late-night salivary cortisol is particularly valuable for catching cyclic Cushing syndrome, where cortisol excess comes and goes over weeks to months. Repeated home collections can capture these fluctuations, which a single office-based test would likely miss. The test also avoids false positives that plague the dexamethasone suppression test in people taking medications that speed up dexamethasone metabolism (like certain seizure drugs).

One limitation: for people with kidney impairment (reduced glomerular filtration rate, a measure of kidney filtering capacity), 24-hour urinary free cortisol becomes unreliable, making salivary testing the better choice. However, kidney function itself affects cortisol levels. Cortisol increases by about 9% for each 10-unit drop in eGFR, so kidney disease can nudge evening cortisol higher and should be factored into interpretation.