Cortisol

Cortisol is the body's main glucocorticoid, meaning the most important hormone in that family of steroids. It is made in the zona fasciculata of the adrenal cortex, which is the outer layer of the adrenal glands. And it is made from cholesterol.

It is not produced on its own: there is a chain of command called the hypothalamic-pituitary-adrenal axis. The hypothalamus, in the brain, releases a hormone called CRH; CRH tells the anterior pituitary, the front part of the gland hanging below the brain, to release ACTH; and ACTH tells the adrenal cortex to make cortisol. And cortisol itself inhibits the secretion of those two hormones, CRH and ACTH: it is as if, on its way out, it slowly turned off the tap it came from.

It also keeps a schedule. Its secretion follows a diurnal rhythm, the rhythm of day and night: it starts to rise during the last hours of sleep, reaches its highest point around the time of waking and drops through the day until it hits its lowest point at night. It is a tide that starts rising before you open your eyes.