Cortisol (S5) - Bedtime

Your body is supposed to power down at night. Cortisol, the main stress hormone, peaks in the morning and falls steadily through the day, reaching its lowest point near bedtime. When that nighttime low disappears, your stress system is stuck in the on position when it should be off.

A bedtime saliva sample captures the active, unbound fraction of cortisol at the moment your body should be quiet. High evening levels show up in research on poor sleep, future diabetes, cardiovascular mortality, and the loss of normal hormone rhythm seen in Cushing's syndrome. It is one of the cleanest windows into how well your stress axis is actually resetting.

What This Test Measures

Cortisol is a steroid hormone made in the outer layer of your adrenal glands and controlled by a chain of signals from your brain (the hypothalamus and pituitary gland) down to those glands. The salivary version of this test measures the free, biologically active fraction of cortisol, while a blood test measures mostly the bound (inactive) form. Because saliva captures only the part of the hormone that can act on your tissues, late-night salivary cortisol has become the preferred way to assess whether the normal nighttime low is intact.

The S5 sample is collected close to bedtime, typically around 22:00 to 23:00. In healthy adults, this should be the lowest cortisol value of the day. Loss of that low point, called the nocturnal nadir, is a biochemical hallmark of cortisol excess and a recurring signal in research on metabolic and cardiovascular risk.

Sleep Quality and Sleep Duration

Bedtime cortisol and sleep are tightly linked. In a 15-day study of young adults wearing sleep-tracking equipment, nights with higher than usual pre-sleep cortisol were followed by shorter sleep, lower sleep efficiency, and longer time to fall asleep. Across individuals, those with shorter average sleep and poorer efficiency tended to have flatter day-to-night cortisol declines.

Inconsistent sleep timing matters too. In a study of 436 midlife adults, more variable sleep timing and duration predicted flatter cortisol slopes, suggesting that erratic schedules and not just total hours can blunt the rhythm.

Type 2 Diabetes Risk

In the Whitehall II cohort of more than 3,000 adults, high bedtime cortisol and a flatter day-to-night decline predicted future impaired fasting glucose or type 2 diabetes. The pattern showed up years before glucose measurements went abnormal, suggesting that elevated evening cortisol is part of the slow drift toward metabolic disease rather than a downstream effect of it.

Cardiovascular Mortality

Within the same Whitehall II population (4,047 adults), people whose cortisol declined less steeply through the day had a higher risk of dying, especially from cardiovascular causes. For each one standard deviation drop in slope steepness, the risk of cardiovascular death rose by about 87% (HR 1.87, 95% CI 1.32 to 2.64). A single morning cortisol value did not carry the same predictive weight, which is why the bedtime end of the curve matters.

Cushing's Syndrome and Hypercortisolism

The most established use of bedtime salivary cortisol is to screen for Cushing's syndrome, a condition where the body chronically overproduces cortisol. In outpatients with suspicion of Cushing's, a bedtime salivary cortisol above 550 ng/dL was 93% sensitive and 100% specific for the diagnosis. A meta-analysis of late-night salivary cortisol in adults found 95.8% sensitivity and 93.4% specificity overall, on par with blood and urine alternatives but easier to collect.

If your bedtime number is consistently elevated, the loss of the nighttime nadir is the signal clinicians look for. It does not confirm Cushing's on its own, but it is one of the strongest non-invasive screens available.

Mental Health and Mood

A meta-analysis of studies in young people found that elevated morning and nocturnal cortisol levels are risk factors for the development of depression in adolescence and young adulthood, supporting a role for an overactive stress axis in mood disorders. Among working adults, higher psychological distress, depression, and burnout symptoms have been linked to higher cortisol from afternoon through bedtime, the opposite of what a healthy rhythm should look like.

Reconciling a Surprising Finding

One study of healthy older adults found that higher bedtime cortisol within an otherwise normal rhythm was linked to better self-rated physical and mental health. This is not a contradiction. Bedtime cortisol is best understood as part of a pattern, not a stand-alone good or bad number. A bedtime value that is high because the entire daily curve sits higher (a sign of HPA dysregulation) is different from a bedtime value that is mildly elevated within an otherwise steep, healthy decline. The shape of the day-to-night drop matters as much as any single point.

Reference Ranges

These ranges come from research populations measured by specific lab methods (LC-MS/MS or immunoassay) and are illustrative orientation rather than universal cutoffs. Your lab will likely report different numbers, possibly in different units. Cushing's screening cutpoints in particular vary by lab method and population (especially in obesity and pregnancy).

Sources: Baid et al. (LC-MS/MS and RIA cutoffs in obese and healthy volunteers); Galm et al. meta-analysis. Compare your results within the same lab over time for the most meaningful trend.

Why One Reading Is Not Enough

Cortisol fluctuates day to day. The within-person biological variation for serum cortisol is roughly 18%, meaning your number can shift meaningfully from one night to the next without anything being clinically wrong. Sampling-design research suggests that at least three days of repeated samples are needed to reliably estimate your average, and even more days if you want a precise read on your day-to-night slope.

Practically, that means treating a single high or low result as a signal to repeat, not a verdict. Get a baseline, retest within 3 to 6 months if you are making changes (sleep schedule, stress management, treatment for a related condition), and then track at least annually. The trajectory tells you more than any single number.

When Results Can Be Misleading

A bedtime cortisol value can be distorted by factors that have nothing to do with the underlying biology. Knowing these helps you avoid a wrong conclusion.

• Sample timing and contamination: collecting too early in the evening, brushing your teeth before sampling (causing trace blood in saliva), or eating or drinking in the 30 minutes before collection can all distort the value.
• Acute illness, surgery, or major stress: any acute physical or psychological insult activates the stress axis and can transiently elevate evening cortisol for hours to days.
• Vigorous late-day exercise: intense exercise in the hours before bedtime can temporarily push cortisol up, even though regular physical activity steepens the daily slope over time.
• Pregnancy and oral estrogens: these change cortisol-binding protein and can reduce the specificity of bedtime salivary readings; interpretation needs context.
• Lab method differences: immunoassays and LC-MS/MS produce different absolute numbers, and immunoassays often show high bias at very low evening levels. Stick with the same lab and method for trending.

What to Do With an Abnormal Result

A single elevated bedtime cortisol is a reason to repeat the test, ideally on two or three separate nights, before making any clinical decision. If the elevation is consistent, the next step is a workup for hypercortisolism. That typically means pairing your bedtime cortisol with a 24-hour urinary free cortisol test (which integrates total daily output) and a 1-mg overnight dexamethasone suppression test (which checks whether your stress axis can be turned off by a synthetic steroid).

If two of those three tests are abnormal, an endocrinologist should be involved to evaluate for Cushing's syndrome, autonomous cortisol secretion from an adrenal nodule, or other causes of HPA dysregulation. If the bedtime value is borderline and your morning cortisol, urine cortisol, and dexamethasone suppression are all normal, the more likely interpretation is functional dysregulation driven by sleep disruption, chronic stress, or shift work, which calls for behavioral and lifestyle intervention rather than endocrine treatment.