When I studied for my masters in Kinesiology 15 years ago I was fascinated by cortisol and the stress response.

Cortisol is a hormone that’s released when we experience stress.
Stress can come in a short burst – like having to swerve quickly in traffic to avoid being hit, to long periods of exercise, working out early in the morning, working out in a fasted state, lack of sleep, to work or life mental worrying. Essentially pushing your body being it’s limits for a long period of time – whether physically, mentally or emotionally.