Does cortisol follow a daily rhythm with a morning peak and low nighttime levels?

The diurnal rhythm of cortisol is a foundational principle of human endocrinology, representing one of the most robust circadian cycles in the body. This 24-hour oscillation is essential for mobilizing energy, regulating immune function, and managing the stress response.

Clinical evidence and patterns

The secretion of cortisol follows a predictable trajectory characterized by significant fluctuations throughout the day:

• The Morning Peak: Upon awakening, individuals experience a sharp rise in cortisol known as the Cortisol Awakening Response (CAR). Peak concentrations typically occur 30 to 60 minutes after waking, generally between 6:00 AM and 10:00 AM. In healthy individuals, these morning levels are the highest of the 24-hour cycle.
• The Diurnal Decline: Following the morning peak, cortisol levels undergo a steady decline throughout the afternoon and evening.
• The Nocturnal Nadir: Cortisol reaches its lowest point (nadir) during the late evening and the first few hours of sleep, typically around midnight. In clinical assessments, midnight serum cortisol levels are expected to be highly suppressed, often measuring below 1.8 μg/dL.

Mechanistic explanations

This rhythmic secretion is primarily governed by the central circadian pacemaker and the hypothalamic-pituitary-adrenal (HPA) axis:

• SCN Regulation: The suprachiasmatic nucleus (SCN) in the hypothalamus acts as the body's master clock, receiving light-dark signals to synchronize physiological processes.
• The HPA Cascade: The SCN coordinates the release of corticotropin-releasing hormone (CRH) from the hypothalamus, which stimulates the pituitary gland to secrete adrenocorticotropic hormone (ACTH). ACTH then signals the adrenal cortex to produce and release cortisol into the bloodstream.
• Aging Considerations: While the fundamental morning-high/night-low pattern persists across the lifespan, factors such as aging and menopause in women can lead to a slight advancement in peak timing or a more "flattened" slope, where the difference between the peak and trough becomes less pronounced.

Bottom line

The claim that cortisol follows a circadian rhythm with a morning peak and nocturnal low is a scientifically established fact. This rhythm is essential for health, and disruptions to this pattern—such as elevated nighttime levels—are often clinical markers for metabolic or HPA axis dysfunction.