Cortisol Night Test

Your body runs on an internal clock, and one of the clearest readouts of that clock is what your cortisol level does at night. Cortisol, your primary stress hormone, is supposed to peak in the early morning and fall to its lowest point around bedtime. When that nighttime drop does not happen, it can be an early signal of hormonal overproduction, a disrupted stress response, or an independent risk factor for heart disease and metabolic problems, even in people who feel fine.

This test specifically checks whether your cortisol rhythm is doing what it should in the evening. A normal morning cortisol reading tells you very little about this, because cortisol is naturally high in the morning in everyone. The nighttime measurement captures something different: whether your adrenal glands (the small glands on top of your kidneys that produce cortisol) are properly winding down. That distinction makes this one of the most sensitive screening tools for conditions like Cushing syndrome, and a surprisingly useful window into cardiovascular and metabolic risk.

What Cortisol Does and Why the Rhythm Matters

Cortisol is a steroid hormone made by the adrenal cortex (the outer layer of each adrenal gland) from cholesterol. Production is controlled by a chain of signals: your brain's hypothalamus releases CRH, which tells the pituitary gland to release ACTH (adrenocorticotropic hormone), which tells the adrenals to make cortisol. This chain is called the HPA axis, and it is the body's central stress-response system.

In healthy people, cortisol follows a tight daily cycle. Levels peak just before waking, giving you the energy to start the day, then gradually decline through the afternoon and evening, reaching their lowest point during the first few hours of sleep. Your brain's master clock, located in a small region called the suprachiasmatic nucleus, controls this rhythm. Nighttime cortisol testing exploits this pattern: if your level is still elevated when it should be at its lowest, something is disrupting the system.

Cushing Syndrome: The Primary Clinical Indication

The most established use of nighttime cortisol testing is screening for Cushing syndrome, a condition in which the body produces too much cortisol. People with Cushing syndrome lose the normal nighttime dip, and catching that loss is the single best way to detect the condition early. Late-night salivary cortisol catches about 96 out of 100 true cases (95.8% sensitivity) and correctly clears about 93 out of 100 healthy people (93.4% specificity). Midnight serum cortisol performs similarly, with 96.1% sensitivity and 93.2% specificity.

Cushing syndrome can be caused by a small benign tumor on the pituitary gland (the most common endogenous cause, accounting for 65 to 70% of cases), an adrenal tumor producing cortisol on its own, or rarely a tumor elsewhere in the body making ACTH. Left untreated, chronic cortisol excess causes weight gain concentrated around the midsection, high blood sugar, bone thinning, muscle weakness, easy bruising, high blood pressure, and mood changes. The mortality risk is 2 to 5 times higher than normal, largely driven by cardiovascular disease and diabetes.

Mild Autonomous Cortisol Secretion

A subtler but far more common condition is mild autonomous cortisol secretion (MACS), previously called subclinical Cushing syndrome. This occurs most often in people with adrenal incidentalomas, which are growths on the adrenal gland found incidentally during imaging done for another reason. These growths show up in 1.4 to 7% of adults, and up to 10% of people over 70. Among people with adrenal incidentalomas, 20 to 50% have MACS.

MACS does not cause the dramatic physical changes of overt Cushing syndrome, but it is far from harmless. People with MACS have higher rates of type 2 diabetes, hypertension (present in about 60% of cases), cardiovascular disease, and spinal fractures compared to people with non-functioning adrenal nodules. In a small randomized trial, surgical removal of the adrenal gland improved or normalized diabetes in 62% of patients, hypertension in 67%, and high cholesterol in 38%. Surveillance alone produced no improvement.

Cardiovascular Risk

Beyond Cushing syndrome, elevated nighttime cortisol is independently linked to heart disease and death from cardiovascular causes, even in people without a hormonal disorder. The evidence comes from several large, long-running studies that tracked thousands of people over years.

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

What this means for you: a nighttime cortisol level that stays stubbornly elevated, even if it does not cross the threshold for Cushing syndrome, may be a cardiovascular warning sign worth tracking. These associations held after adjusting for age, sex, smoking, BMI, and other standard risk factors, suggesting the cortisol pattern itself carries independent information.

Diabetes Risk

The same pattern shows up in diabetes risk. In the Whitehall II cohort, 3,270 adults with normal blood sugar were followed for 10 years. Those with higher evening cortisol were about 18% more likely to develop type 2 diabetes per standard-deviation increase (OR 1.18). In a separate study of 1,478 hypertensive adults with obstructive sleep apnea, higher midnight cortisol was associated with a 25% increase in type 2 diabetes risk per standard-deviation increase (HR 1.25), independent of standard diabetes risk factors.

If you are tracking your metabolic health with fasting glucose or HbA1c, a persistently elevated nighttime cortisol could help explain why your blood sugar is creeping up despite good lifestyle habits. Cortisol directly raises blood sugar and promotes insulin resistance, so a nighttime cortisol that does not drop may be quietly undermining your metabolic health from behind the scenes.

Cancer Survival

Two studies have linked disrupted nighttime cortisol patterns to shorter survival in cancer patients. In 113 women with ovarian cancer, each standard-deviation increase in nighttime cortisol was associated with a 46% greater mortality risk, and those with the highest nighttime cortisol survived an average of 3.3 years compared to 7.3 years in the lowest group. In 104 women with metastatic breast cancer, a flattened daily cortisol rhythm (meaning cortisol did not drop normally at night) predicted earlier death, and this was linked to reduced natural killer cell activity, part of the immune system's ability to fight cancer.

Reference Ranges

Nighttime cortisol thresholds depend heavily on which assay your lab uses. Values established using one method cannot be applied to results from a different method, so always compare your results within the same lab over time. Age also matters: cortisol increases by roughly 11% for each 10-year increase in age, and people over 50 have meaningfully higher nighttime levels than younger adults.

These thresholds are drawn from published research and clinical guidelines. Your lab may use different assays and cutpoints. Compare your results within the same lab over time for the most meaningful trend.

From a longevity perspective, no professional society has established formal "optimal" targets for nighttime cortisol below the pathological thresholds. However, the Leiden Longevity Study found that offspring of people who lived past 90 had lower evening salivary cortisol (3.32 nmol/L) compared to their partners (3.82 nmol/L). While this does not establish a target, it does suggest that lower nighttime cortisol tracks with longer life.

Tracking Your Trend

A single nighttime cortisol reading is not enough to make any decision on. This biomarker has some of the highest test-to-test variability of any hormone measurement: the intra-individual coefficient of variation is about 22% in healthy people, 32% in people being evaluated for Cushing syndrome, and as high as 51% in confirmed Cushing patients. Up to 50% of people show meaningfully different results on consecutive samples.

Guidelines recommend collecting at least 2 to 3 samples before drawing any clinical conclusion. For preventive tracking, get a baseline reading, then retest in 3 to 6 months if you are making lifestyle changes that might affect your cortisol rhythm (such as improving sleep, managing stress, or adjusting meal timing). After that, annual testing gives you a trend line that is far more informative than any single number. A gradually rising nighttime cortisol over several years could flag a developing problem long before symptoms appear.

When Results Can Be Misleading

Because nighttime cortisol is exquisitely sensitive to timing and context, several common situations can produce a reading that does not reflect your true baseline.

• Shift work or irregular sleep schedules: the test assumes cortisol should be low in the late evening. If you routinely go to bed after midnight or work night shifts, the result is uninterpretable. Collect the sample at your usual bedtime, not at a fixed clock time.
• Acute stress or hospitalization: being stressed, anxious, or in an unfamiliar environment can spike cortisol. Ideally, collect salivary samples on a quiet evening at home. If serum cortisol is drawn in a hospital, you need at least 48 hours of admission before the result is reliable.
• Kidney function: impaired kidney function (eGFR below 60 mL/min/1.73m²) is associated with higher cortisol levels. Cortisol increases by roughly 9% for each 10 mL/min/1.73m² decrease in eGFR, which can push results above diagnostic thresholds without true Cushing syndrome.
• Contamination: topical hydrocortisone creams, oral steroid gels, and even licorice or chewing tobacco (which contain a compound that interferes with cortisol metabolism) can falsely elevate salivary cortisol. Avoid eating or drinking for 2 hours before collection.