The HPA axis explanation states that short term stress can be beneficial. However, prolonged exposure to stress results in the HPA axis remaining active. This can be unhealthy because .

Prolong exposure to stress causes a continuous production of cortisol, which can lead to depression and fatigue.

Explanation:

HPA axis is the hypothalamic–pituitary–gonadal axis. HPA axis is often taken as a body;s stress system, which controls the amount of cortisol and other stress hormones.

Prolong exposure to stress, increases the cortisol production in body. Prolong stress can also cause certain changes to the neurons located in paraventricular nucleus, ultimately resulting in sustained activation of the axis. Long term activation of the axis causes negative feedback mechanism and ultimately causes fatigue, exhaustion, headache and depression etc.  

steady state because it the only on that maes sense