Our bodies are set up to get excited when things get difficult. It’s a natural response, and a good one. Under threat, our pulses quicken, our appetite reduces, we eject unnecessary digestive weight, and we become alert and sensitive. This enhances our ability to handle things in the short term. However, the longer term effects of excitement under difficulty are not good. Our immune, circulatory and other systems can suffer, and we can get exhausted.
You can read a bit more about the adrenaline response here (article by Healthline).
Life is a balance between extreme relaxation and extreme excitation. Neither end of the spectrum is healthy in excess. Our organisms work best in fairly rhythmic wave movements between wakefulness and sleepiness, activity and rest.
Regular exercise is a very safe, stress-reducing way to indulge in wakeful activity (read more here from Harvard Health). During exercise, adrenaline and cortisol levels increase; but, as a reaction, resting adrenaline and cortisol is reduced. This is similar to the way that exercise, by temporarily increasing heart rate, ultimately reduces resting heart rate.
If we don’t exercise, then our body does not have a regular, safe way to process adrenaline. We are more likely to experience such fight-or-flight hormones in our daily interactions with people, causing relational stress. In other words, exercise conditions us to be more chilled when we are not exercising.
The takeaway from this is: if we want to have low-stress lives, then it can be transformative to take regular exercise which raises our heart rate. Half an hour a day can be enough (see this link from Mayo Clinic).