Cortisol (U4 Bedtime) Test

Your body is supposed to do something specific at bedtime: turn down the volume on the main stress hormone, cortisol. A healthy nighttime reading is low. A high reading suggests your stress system is still running when it should be resting, which is one of the earliest signals that something is off with sleep, recovery, or metabolic health.

This test captures cortisol in a dried urine sample collected at bedtime (U4), the final point in a four-sample daily curve. Bedtime is the natural low point of the daily cortisol rhythm, so it is the most sensitive moment to detect a system that is not switching off properly.

What This Test Actually Measures

Cortisol is a steroid hormone made by your adrenal cortex, the outer layer of small glands sitting on top of your kidneys. Its release is controlled by a chain of signals scientists call the HPA axis (the hypothalamus-pituitary-adrenal axis), which is essentially your brain's command line to the adrenals.

Cortisol follows a daily clock. It surges in the early morning to help you wake and mobilize energy, drifts down through the afternoon, and reaches its lowest point near bedtime. A urine sample collected at bedtime captures the free, biologically active cortisol your kidneys filter out during this winding-down phase. When that nighttime number is high, it means the off-switch is not working.

Why a High Bedtime Reading Matters

A flatter daily cortisol curve, where bedtime levels stay too high, is consistently linked to worse mental and physical health. A meta-analysis covering 80 studies found a small to moderate overall link between flatter daily slopes and poorer outcomes, with the strongest effects on immune and inflammatory measures. In plain terms: when your nighttime cortisol does not fall the way it should, your inflammatory system tends to run hotter.

Bedtime cortisol elevation has been observed across several conditions involving stress-system disruption. The size of the elevation, and whether the daily rhythm is preserved, helps separate everyday stress from more serious endocrine disease.

Cushing's Syndrome

The most clinically established use of late-night cortisol testing is screening for Cushing's syndrome, a condition of chronic cortisol excess. In this condition, the normal day-night rhythm is often lost entirely. Late-night salivary cortisol testing has shown sensitivity around 92 to 96 percent and specificity around 93 to 96 percent for detecting Cushing's syndrome across multiple studies, making it one of the strongest first-line screening tools for the disease. Most of this evidence comes from saliva rather than dried urine, but the underlying biology of the bedtime low point is the same.

Type 2 Diabetes and Metabolic Health

In a community-dwelling cohort, people with type 2 diabetes showed a flatter daily cortisol decline and higher bedtime cortisol than those without diabetes. The pattern was modest, not the dramatic elevation seen in Cushing's, but it pointed to a measurable disruption in stress-hormone control among people with established metabolic disease.

Mood, Sleep, and Psychiatric Conditions

Higher bedtime cortisol has been documented in people with bipolar disorder, and in that group, elevated bedtime levels have been linked to past suicide attempts in a study of 165 participants. A meta-analysis on adolescents and young adults found that elevated morning and nighttime cortisol were risk factors for developing depression, supporting a role for stress-system overactivity in mood disorders.

Night Shift Work and Circadian Disruption

Female night-shift hospital workers showed a flattened, U-shaped cortisol curve with higher bedtime cortisol after night shifts, when levels would normally be low. In a separate study of female healthcare workers, night and long shifts were significantly associated with impaired morning and bedtime cortisol patterns. If you work irregular hours, your bedtime number is likely to reflect the cost.

What a Low Bedtime Reading Can Mean

While most attention goes to high values, an abnormally low reading paired with low cortisol throughout the day can suggest primary adrenal insufficiency (also called Addison's disease), where the adrenal glands underproduce cortisol. Diagnosis of this condition relies primarily on morning testing, but a flat low pattern across all time points is a clue worth investigating. A low daily HPA-axis output has also been documented in some studies of hypertensive and coronary heart disease patients.

Reference Ranges

There is no single universally agreed cutpoint for bedtime urinary cortisol. Most published reference data come from late-night salivary or serum cortisol used to screen for Cushing's syndrome, and they vary by assay and population. The numbers below are illustrative orientation, not universal targets. Your lab will likely report different numbers, possibly in different units. Compare your result within the same lab over time.

What this means for you: a single bedtime number is hard to interpret in isolation. The shape of your full daily curve, especially the difference between waking and bedtime values, carries more information than any one point alone.

Why One Reading Is Not Enough

Cortisol fluctuates from day to day, hour to hour, and even minute to minute. A study of cortisol variability in Cushing's disease found extreme swings in late-night levels in some patients, which can blur a single test result. Even in healthy people, a stressful day, a poor night of sleep, or a recent illness can shift one bedtime number well outside your usual range.

Get a baseline reading. If your bedtime value looks elevated, retest in 3 to 6 months, ideally collecting samples on a few consecutive days to capture the trend rather than a single moment. If you are working on sleep, stress management, or shift-work recovery, retesting every 6 months gives you a way to see whether your interventions are actually quieting the system. Compare your numbers against your own prior values from the same lab, not against generic population ranges.

When Results Can Be Misleading

Several common factors can push a bedtime cortisol reading away from your true baseline. Knowing these in advance helps you collect a sample that reflects your real biology, not a one-day artifact.

• Acute stress or illness: Major surgery, infection, and critical illness can drive cortisol up to about four times normal levels and disrupt the daily rhythm for at least a week afterward. Wait until you have recovered before testing.
• Shift work or recent travel: Night shifts flatten the daily curve and raise bedtime cortisol. If you are testing during or right after a stretch of night shifts or time-zone travel, the result reflects your work schedule, not your baseline stress system.
• Obesity: In people with obesity, bedtime cortisol can be modestly elevated for reasons unrelated to true cortisol excess, which lowers the test's specificity. A high reading in this context warrants confirmation rather than alarm.
• Lab assay differences: Immunoassays can produce different values than mass spectrometry methods for the same sample. Always compare your results within the same lab over time, since cutoffs are not interchangeable across methods.

What to Do With an Abnormal Result

A single elevated bedtime cortisol does not equal a diagnosis. The next step is to put it in context. If your result is high and your full daily curve is also flattened (high evening, blunted morning peak), that pattern points toward sustained HPA-axis activation worth investigating. If your reading is markedly high and rhythm is lost, consider an endocrinology consultation to screen for Cushing's syndrome with confirmatory testing such as a dexamethasone suppression test or 24-hour urinary free cortisol.

If your bedtime reading is mildly elevated without dramatic rhythm loss, the more likely culprits are sleep disruption, shift work, chronic stress, or metabolic factors. In this case, the next steps are practical: optimize sleep timing, address sources of chronic stress, and pair this test with markers of metabolic health (fasting glucose, HbA1c, insulin) and inflammation (hs-CRP) to see whether stress-system overactivity is showing up elsewhere in your biology.