What is the Origin of Cortisol Diurnal Variation?

The term combines cortisol, the name of the stress hormone, with diurnal, derived from the Latin diurnalis, meaning "of the day," and variation, referring to the change over time. This concept is foundational to endocrinology, reflecting the evolutionary need for a hormonal rhythm that prepares the body for the metabolic and cognitive demands of the active day phase. The rhythm is intrinsically linked to the light-dark cycle.

What is the Mechanism of Cortisol Diurnal Variation?

The Hypothalamus-Pituitary-Adrenal axis orchestrates this rhythm, with the suprachiasmatic nucleus (SCN) of the hypothalamus acting as the master clock. In the early morning, the SCN signals the hypothalamus to release CRH, which stimulates the pituitary to release ACTH, culminating in the adrenal glands producing the characteristic cortisol surge. As the day progresses, negative feedback loops and decreasing SCN drive lead to the hormone's nadir during the late evening, facilitating the transition to sleep.

Cortisol Diurnal Variation

Meaning

Cortisol diurnal variation describes the predictable, 24-hour rhythmic fluctuation in the secretion of the glucocorticoid hormone cortisol from the adrenal glands. This natural pattern is characterized by a sharp peak shortly after waking, a gradual decline throughout the day, and the lowest levels occurring around midnight. Clinically, evaluating this variation is more informative than a single random measurement, as it reveals the functional integrity of the Hypothalamic-Pituitary-Adrenal (HPA) axis and the body’s stress adaptation capacity. Deviations from this established curve are hallmarks of chronic stress and hormonal imbalance.