Cortisol

Cortisol is a hormone made by the two adrenal glands (located one on each kidney) and is essential for life. It is usually referred to as the "stress hormone" as it is involved in response to stress and anxiety. It increases blood pressure and blood sugar, and reduces immune responses. The amount of cortisol released by the adrenal glands is regulated by the pituitary gland inside the brain.

The amount of cortisol present in the blood undergoes diurnal variation, with the highest levels present in the early morning, and the lowest levels present around midnight, or 3-5 hours after the onset of sleep.