What is the Origin of Cortisol Diurnal Curve?

The recognition of cortisol's rhythmic, day-night variation is a core principle of chronobiology and endocrinology, established through decades of physiological research. The term 'diurnal' explicitly refers to the daily cycle, contrasting with pulsatile or ultradian rhythms. Understanding this curve is paramount in clinical practice for evaluating stress response integrity and identifying potential adrenal dysfunction.

What is the Mechanism of Cortisol Diurnal Curve?

The suprachiasmatic nucleus (SCN) in the hypothalamus, the body's central clock, orchestrates this rhythm by influencing the release of Corticotropin-releasing Hormone (CRH). CRH stimulates ACTH from the pituitary, which in turn drives cortisol production from the adrenal glands in a time-dependent manner. This mechanism ensures that peak cortisol, a hormone critical for wakefulness and metabolism, aligns with the beginning of the active day cycle.

Cortisol Diurnal Curve ∞ Area

Cortisol Diurnal Curve

Meaning

The predictable, cyclical pattern of cortisol secretion observed over a 24-hour period, reflecting the entrainment of the Hypothalamic-Pituitary-Adrenal (HPA) axis to the circadian rhythm. A healthy curve is characterized by a sharp rise shortly after waking (the Cortisol Awakening Response or CAR), peaking in the early morning, and gradually declining throughout the day to its lowest point near midnight. Clinical assessment of this curve is a crucial diagnostic tool for HPA axis function.