Cortisol

A steroid hormone released by the adrenal gland as part of the hypothalamus-pituitary and gland-adrenal gland (HPA) axis in response to prolonged stress, via mobilization of energy reserves and immunity suppression.

Reference:

Bear, M.F., Connors, B.W. & Paradiso, M.A. (2001). Neuroscience: Exploring the Brain. (2nd Edition). Baltimore, MD: Lippincott Williams & Wilkins.