cortisol
Cortisol is the body's main stress hormone — the chemical that helps you cope when life gets demanding, whether that means waking up in the morning, recovering from an injury, or surviving a famine. Made by the adrenal cortex, it is sometimes called the body's natural steroid, and it touches almost every tissue. A good mental image is a master metabolic dial that the body turns up to mobilize energy and dampen distractions when resources matter most.
Biochemically, cortisol is the principal glucocorticoid. It raises blood glucose by promoting gluconeogenesis in the liver and by making fat and muscle release fuel, ensuring the brain has sugar during stress. It also has powerful anti-inflammatory and immune-suppressing effects, which is why synthetic versions are used as medicines. Cortisol enters cells and binds an intracellular receptor that changes which genes are switched on.
Cortisol is released in a daily rhythm — highest in the early morning and lowest around midnight — under the control of ACTH and the HPA axis. The rhythm matters: chronic, around-the-clock elevation (whether from disease or from medication) produces the harmful effects seen in Cushing's syndrome, including muscle wasting, thin skin, high blood sugar, and weakened immunity.
When a clinician suspects too little or too much cortisol, they often measure it in the early morning, because a low morning value can flag adrenal insufficiency while a value that fails to fall after dexamethasone suggests Cushing's syndrome.
Cortisol's daily rhythm is why timing matters in testing.