Cortisol (Night)

Late-night salivary cortisol has one main job: show whether the nighttime low point is still there. Cortisol should be high near waking and low near sleep. Cushing syndrome breaks that pattern. The night sample is built to catch that failure.

This is an established endocrine test when endogenous Cushing syndrome is suspected. Endogenous means the excess comes from your own adrenal system, not from steroid medicines. It is much weaker as a general stress test, because one rough evening can move a saliva result without proving a cortisol disorder.

What It Measures

Cortisol is a steroid hormone released by the adrenal glands after signals from the brain. In blood, most cortisol rides on carrier proteins. Saliva mainly reflects free cortisol, the part that can move into tissues at that moment.

This version is commonly run by an automated antibody-based lab method. That method tracks well with more specific lab methods, but nighttime cortisol is low enough that small method differences matter. If you repeat the test, comparing results from the same lab method is cleaner.

Why the Night Sample Matters

In a stable sleep schedule, cortisol reaches its low point late in the evening or around sleep. People with Cushing syndrome often lose that low point. That is why guidelines list late-night salivary cortisol beside 24-hour urine free cortisol and dexamethasone suppression testing as first-line options for suspected Cushing syndrome. Of these first-line screens, late-night salivary cortisol is generally the most specific.

In a pooled adult meta-analysis, late-night salivary cortisol caught 92% of Cushing cases and correctly ruled out 96% of non-cases. Across the broader literature, sensitivity is typically 92% or higher and specificity 93% or higher. The exact performance depends on the lab method and on who is being tested. A high result sorts who needs confirmation. It does not tell you where the excess cortisol is coming from.

What a High Result Should Trigger

A high night result should be checked with a second late-night saliva sample or another Cushing screen, such as 24-hour free cortisol or an overnight dexamethasone suppression test. Dexamethasone is a steroid that should switch cortisol production down overnight. If cortisol does not fall, the finding supports cortisol excess.

If repeat testing still points high, the next step is source-finding. ACTH is the pituitary signal that tells the adrenal glands to make cortisol. Pairing ACTH with cortisol testing helps separate a pituitary or other signaling source from an adrenal source. Imaging comes after biochemical confirmation, not before.

When It Is Just Noise

A late-night reading can rise after acute stress, disrupted sleep, or a collection mistake. That does not make it useless. It means the test should be used for the question it was built for: loss of the nighttime cortisol low point in a Cushing workup.

If your question is burnout, poor sleep, or chronic stress, a full-day saliva curve is more useful than one night point. Even then, the curve is a pattern marker, not a diagnosis of why you feel stressed.

When Results Can Be Misleading

• Sleep timing: shift work, jet lag, changing bedtimes, or collecting far from your usual bedtime can make clock time and body time disagree. Sampling at your usual bedtime, rather than a fixed midnight clock time, reduces false positives.
• Steroid exposure: pills, injections, inhalers, creams, and oral steroid gels can either change your own cortisol production or contaminate the sample.
• Mouth contamination: brushing, flossing, bleeding gums, mouth sores, food, alcohol, smoking, or chewing tobacco near collection can distort the result.
• Licorice: real licorice can raise salivary cortisol for several days to a few weeks by slowing the local conversion of cortisol to cortisone.
• Physiologic activation: severe illness, heavy alcohol use, major depression, and acute stress can produce Cushing-like chemistry without a cortisol-producing tumor.

Low Night Cortisol

Low cortisol at night is usually expected. It is not the main way to check adrenal insufficiency. Adrenal insufficiency is when the body cannot make enough cortisol when it needs it. Morning blood cortisol with ACTH stimulation testing is the better-matched workup, and some centers now use waking saliva steroid measurements during stimulation testing, especially when estrogen pills make blood cortisol harder to read.

How to Use It Over Time

Trends help only when the setup is repeatable: same lab method, same relation to bedtime, stable sleep schedule, and no recent steroid exposure. A changing bedtime can look like changing biology.