In simplest terms, cortisol is your body's main stress hormone. Your adrenal glands pump it out every time you are alarmed. Everytime your body gets alarmed by a certain situation, that's cortisol at work.
Although, people often associate this hormone with negative things, cortisol plays an essential role in our lives. A certain amount of cortisol is healthy for the body, as it has anti-inflammatory properties and it's