Normal Evening Cortisol Levels in Healthy Adults
In healthy adults, normal evening cortisol levels should be <1.8 μg/dL (<50 nmol/L), with midnight serum cortisol specifically <1.8 μg/dL in sleeping individuals. 1
Evening/Nighttime Reference Values
Midnight serum cortisol in sleeping individuals should be <50 nmol/L (<1.8 μg/dL), representing the physiologic nadir of the circadian rhythm 1
Late-night salivary cortisol (collected at bedtime, typically 11 PM-midnight) should be <3.6 nmol/L in healthy adults, with values above this threshold suggesting abnormal cortisol secretion 1, 2
Evening cortisol levels at 10:00 PM in healthy adults aged 21-60 years should be <1.6-1.9 μg/L based on salivary measurements 3
Physiologic Context
The cortisol nadir is tightly entrained to sleep onset, occurring at the patient's usual bedtime rather than strictly at midnight 4
Normal individuals demonstrate a clear circadian rhythm with morning levels of 5-23 μg/dL (138-635 nmol/L) that progressively decline throughout the day to reach their lowest point at night 1
Loss of this normal evening nadir is a hallmark of pathologic hypercortisolism, making evening measurements particularly useful for detecting Cushing's syndrome 2, 3
Critical Considerations for Interpretation
Timing matters: Evening cortisol should be measured at the patient's usual bedtime when cortisol should be at its nadir, not necessarily at a fixed clock time 4
Sleep requirement: The midnight serum cortisol reference value of <1.8 μg/dL specifically applies to sleeping individuals, as the nadir is linked to sleep onset 1
Night-shift workers: Standard evening cortisol measurements are unreliable in individuals with disrupted sleep-wake cycles or night-shift workers, as their circadian rhythm is altered 1, 4
Stress effects: Acute psychological or physical stress can transiently elevate cortisol at any time of day, potentially invalidating evening measurements 1