The right way of understanding stress is to look at its evolotion. The causes of stress lie in our evolutionary past.
While understanding stress is good, stress itself is neither good nor bad. The key to understanding stress is to ask, “Why did stress evolve?”
When we are stressed, our bodies release a burst of hormones (called “glucocorticoids”) that prime the body for flight or fight. If you imagine one of your ancestors 100,000 years ago unexpectedly encountering a large carnivore, obviously becoming quickly ready either to flee or fight was much more advantageous than only becoming slowly ready!
The stress hormones have a negative-feedback loop. In such a loop, the production of a lot of X causes less of X to be produced and the production of a little of X causes more of X to be produced. So, when the body is awash in stress hormones, it produces fewer of them.
Apparently there is a variety in the number of “receptors” for stress hormones in the hippocampus areas of mammalian brains. The more receptors there are, the greater sensitivity there is to stress hormones. Someone with a lot of receptors will shut down the production of stress hormones much more quickly than someone with fewer receptors.
It’s very interesting to note that the number of receptors is not genetically set, but, instead, it is heavily influenced by an animal’s environment. This is a plasticity that occurs naturally.
Parents pass traits to their offspring in two ways: (1) genetically and (2) behaviorally. A specific gene can be silent or very active, and the difference is environmental. This is why your genes don’t determine your destiny. How your environment (for example, the behavioral patterns of your parents) affects you alters the expression of your genes and, so, your long-term temperament and behavior.
Mother Nature wants to equip animals to survive and reproduce. Normally, most animals spent most of their lives in environments very similar to those they were born into. Animals that grow up in an enriched, nurturing environment will have a muted response to stress due to the development of plenty of receptors in their brains; on the other hand, animals that grow up in an impoverished environment (for example, with fearful mothers) will be easily stressed because fewer receptors developed in their brains.
If the environment remains relatively unchanged, those animals with a more muted response to stress will flourish. However, if the environment changes dramatically, those animals who are more easily stressed will have a reproductive advantage.
Chronic stress is a problem. If you have chronically high levels of stress hormones, you are likely to suffer such ill effects as increased blood sugar and fats, disrupted sleep, abnormal cognitive function, and abnormal emotional function. This will predispose you to chronic illnesses such as cardiovascular disease and diabetes.
Nevertheless, there are environments, such as long periods of starvation, in which chronically high levels of stress hormones enable some animals to enjoy an evolutionary advantage.
This explains why stress itself is neither good nor bad.
Still, the popular emphasis on stress reduction methods is not misplaced in our environment. The levels of noise, incessant distractions, and various common threats increase our risk of suffering chronic illnesses such as cardiovascular disease and diabetes.
This is itself a sufficient reason for you to adopt a daily breathing practice such as zazen if you haven’t already done so.