Cortisol is the primary glucocorticoid hormone produced by the adrenal cortex. It plays a central role in the stress response, mobilising glucose and fatty acids for energy, regulating immune activity, and maintaining blood pressure. Cortisol follows a strong circadian rhythm, peaking in the early morning and falling to its lowest around midnight.

This diurnal pattern is clinically important: morning cortisol assesses adrenal sufficiency, while evening cortisol investigates Cushing's syndrome (cortisol excess). Chronic stress, sleep disruption, and various medical conditions alter this normal rhythm, with consequences for metabolism, immune function, and mental health.