In the hypothalamic-pituitary-adrenal (HPA) axis, the CRH-ACTH-cortisol system exhibits a circadian rhythm. The peak cortisol secretion normally occurs at approximately:
- A Midnight (00:00 hours)
- B 4–8 AM (early morning hours) ✓
- C 2–4 PM (mid-afternoon)
- D 8–10 PM (evening)
Correct answer: B. 4–8 AM (early morning hours)
Explanation
Cortisol secretion is driven by pulsatile CRH and ACTH release and follows a circadian pattern entrained to the light-dark cycle. Peak cortisol levels occur in the early morning (4–8 AM), approximately 1–2 hours before waking. Nadir cortisol is around midnight. This rhythm is driven by the suprachiasmatic nucleus via the HPA axis. Disruption of this rhythm (as in Cushing's disease or shift work) is clinically significant; loss of diurnal variation is a diagnostic clue for Cushing's syndrome.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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