Cortisol is closely linked with stress. It is a hormone that works as a key player in the body's stress response and is often measured in research as an indicator of stress. Cortisol plays a vital role in the body's functioning; it's secreted by the adrenal glands and involved in the following functions and more:
    Proper glucose metabolism
    Regulation of blood pressure
    Insulin release for blood sugar     
    maintenance
    Immune function
    Inflammatory response

It can also fluctuate based on what a person is experiencing. For instance, although stress isn’t the only reason that cortisol is secreted into the bloodstream, it has been termed “the stress hormone” because it’s also secreted in higher levels during the body’s stress response and is responsible for several stress-related changes in the body. Small increases of cortisol have some positive effects:

    Heightened memory functions
    A burst of increased immunity
    Lower sensitivity to pain
    Helps maintain homeostasis in the body