A note for Lesson no. 1, "You Have One Brain (Not Three)," in Lisa Feldman Barrett.
Some context from page 26 is:
...cortisol, a hormone that provides a quick burst of energy.
Contrary to popular belief, cortisol is not a stress hormone. It is secreted during moments of stress but also at other times, like when when you wake up in the morning or when you exercise. Cortisol gets glucose into your bloodstream quickly and enhances your cells to use that glucose for fuel, to produce a quick burst of energy when you need one. You always have cortisol in your bloodstream, whether you are stressed or not, and your brain regulates the amount throughout the day to help keep your metabolism running efficiently.
If cortisol is secreted frequently when it is not needed, the excess cortisol can contribute to illness. An example is "chronic stress," when your brain predicts a need for a big burst of energy that end ups being unnecessary,