Cortisol is more than just a stress hormone; its primary role is to tell other body systems when it’s night and day, so that they can function appropriately and in a synchronized fashion. Cortisol works in partnership with its night-time counterpart melatonin, a hormone secreted from the pineal gland in response to darkness. Cortisol and melatonin are counter-regulatory, thus if cortisol is high at night, it will inhibit melatonin secretion during sleep.