Cortisol, deemed the quintessential stress hormone, allows us to cope with important events and imminent threats. A spike in cortisol levels mobilizes necessary resources—such as by tapping into our body’s reserves to produce energy—and then allows us to return to a stable state. But can our bodies cope with prolonged or repeated stress in the same way? Some studies report lower cortisol levels in humans—or other mammals—subject to chronic stress, while other studies contradict these findings. In light of this, is cortisol still a reliable stress indicator?