Cortisol is secreted in a diurnal pattern. In healthy individuals, the peak secretion of cortisol occurs at what time relative to waking?

• A At midnight
• B Shortly before waking (early morning) ✓
• C Around noon
• D Late evening

Explanation

Cortisol follows a circadian rhythm driven by hypothalamic CRH pulses synchronized with the suprachiasmatic nucleus. Plasma cortisol peaks in the early morning hours (approximately 6–8 AM), just before and around the time of waking. It reaches its nadir in the late evening. Loss of this diurnal variation is a hallmark of Cushing's syndrome, where midnight cortisol remains elevated.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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