Cortisol PM low — diurnal rhythm and what it means
Cortisol follows a strong diurnal rhythm: peak ~30 min after waking (cortisol awakening response), trough at midnight. A typical 4 p.m. cortisol is ~3-10 µg/dL; a midnight cortisol is typically <2 µg/dL. PM cortisol that is appropriately low confirms the diurnal rhythm is intact. The interesting research-context patterns are flattened rhythms — where AM cortisol is also low and the morning-to-evening drop is muted — which the literature associates with chronic stress exposure, depression, PTSD, and shift work, not with the popular 'adrenal fatigue' framing. Single-point PM cortisol alone is rarely actionable; the diurnal slope is the meaningful number.
Reference ranges
| Morning (8 a.m.) reference | 5 – 25 µg/dL |
| Afternoon (4 p.m.) reference | 3 – 10 µg/dL |
| Evening (10 p.m.) reference | < 5 µg/dL |
| Late-night salivary (midnight) | < 0.15 µg/dL |
| AM:PM diurnal slope (research) | > 3:1 healthy |
What this marker measures
PM cortisol on its own is a low-information data point. The meaningful test is a 4-point diurnal salivary cortisol profile (waking, +30 min, afternoon, bedtime), which reveals the slope and any disruption. Late-night salivary cortisol specifically is the screening test for Cushing's syndrome (elevated late-night cortisol, even with normal AM, indicates loss of diurnal rhythm).
Why might it be low?
- ·Healthy diurnal rhythm — appropriately low evening
- ·Adrenal insufficiency (also low AM)
- ·Pituitary insufficiency
- ·Chronic glucocorticoid use (HPA suppression)
- ·Severe HPA dysregulation in chronic stress (debated literature)
Why might it be elevated?
- ·Cushing syndrome (loss of diurnal rhythm)
- ·Acute stress / illness
- ·Depression with HPA hyperactivity
- ·Recent venepuncture stress
- ·Late-day caffeine intake
FAQ
Is "adrenal fatigue" real?+
Not as a recognised endocrine diagnosis. The popular framing — that chronic stress 'exhausts' the adrenals and produces low cortisol output — is not supported by mainstream endocrinology. The actual physiology in chronic stress is HPA-axis dysregulation, often with disrupted rhythm rather than overall low output. True adrenal insufficiency (Addison disease) is a discrete clinical entity with autoimmune or other specific causes.
How do I get a real diurnal profile?+
4-point salivary cortisol kits (DUTCH test, ZRT) collect saliva at waking, +30 min, afternoon, and bedtime. Salivary cortisol reflects the free (bioactive) fraction. The test is more informative than any single serum draw and is widely used in functional-medicine and longevity-oriented assessments.
This page describes biomarker research and reference ranges for self-tracking and research-context discussion only. It is not medical advice, not a diagnosis, and not a substitute for a qualified physician. Take any concerns about your health to a clinician.
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