To help explain how the animals, including humans, adapt to stress, Dr. Hans Selye developed the general adaptation syndrome seen below.
This figure depicts the 3 phases of adaptation to stress and how an individual's resistance to stress changes over time. In the first phase, Alarm Reaction, a stressor is experienced and the initial reaction is a decreased resistance to stress. This is because the stressor startles the person experiencing stress, but this changes very quickly, in the blink of an eye. In an instant, the sympathetic branch swings in to action to prepare the individual to deal with the stressor leading in to the second phase, Resistance.
During the Resistance phase, the individual has an increased resistance to stress as the sympathetic branch places them on high alert. If the stress ends before resources are depleted, the parasympathetic branch will begin to swing in to action to help the person recover the resources used during the resistance phase. You don't typically run in to problems unless the stress is unabating. Eventually, if the stress is not resolved, the individual will enter the exhaustion phase which leaves them at a reduced ability to deal with stress. This is where they are forced in to a parasympathetic state.
Now, given the name General Adaptation Syndrome, you may be able to figure out that this is the way the body responds to all stress. In other words, even if you are able to defeat one stressor, all of the other stress that you are under can still force you in to the exhaustion phase. In addition, whether the stress is physical or psychological is irrelevant, this same process occurs whether you are fighting off a lion or fretting over whether you can make your mortgage. This system is optimized to work with stress being experienced intermittently, it doesn't work so well when stress is chronic and never-ending.
Given what we know about the 2 branches of the autonomic nervous system and looking at the General Adaptation Syndrome, you can see that being in a sympathetic state day in and day out is not a good place to be. Eventually your resistance to stress will tank and the slightest stress will set you off and force you back in to the exhaustion phase until you accumulate enough resources to deal with another stressor. Since the pressures of work and life tend to accumulate very easily, it becomes important to make sure you take the time to incorporate some activities that activate the parasympathetic branch of the autonomic nervous system to help your body recover from the rigors of daily life.
Most people are familiar with the benefits of exercise so they often undertake programs such as weight training, distance running, or some other physically demanding activity to keep healthy. The problem is, these activities are sympathetic branch activities so they add to sympathetic activity. There are countless benefits to exercise, so I'm not suggesting you avoid it. What you should do is incorporate other activities such as yoga, stretching, massage, foam rolling, meditation, or even steam room/sauna heat therapies that increase parasympathetic nervous system activity to help you manage your stress by increasing your stress resistance.
Other lifestyle factors can also have a big impact on your ability to deal with stress via the autonomic nervous system. Sleep is crucial to helping your body recover from and deal with the stress of everyday life. One not so obvious activity may also have a pretty significant impact on your ability to deal with stress: Reducing sedentary time.
While you may think of sitting down as a parasympathetic activity, which it is, the issue is actually a little more complicated than that. While you want to incorporate parasympathetic activities to help you deal with stress, your perception of stress is equally important. Recent research in inactivity physiology has identified changes in the brain that are associated with a sedentary lifestyle.
In studies done in rats, researchers have shown that high levels of sedentary behavior lead to changes in the brain in an area important to the regulation of sympathetic nervous system activity, the rostral ventrolateral medulla(RVLM)(1). High levels of inactivity are associated with increased branching of neurons in the RVLM that increase sympathetic nervous system activity(2). This increased branching of nerves would theoretically increase sympathetic nervous system activity for a given stressor and likely lower the threshold at which sympathetic nervous system activity is increased leading lower stress resistance and a shorter time to the exhaustion phase.
This shows that the relationship between stress and health is not about keeping stress as low as possible, but more about experiencing a sweet spot for stress that is neither too low nor too high. This means that you want to balance activities between sympathetic and parasympathetic activity dominant, not drop sympathetic activities as low as possible while increasing parasympathetic activities as high as possible. This means that most people will have to increase parasympathetic activities such as yoga and meditation while reducing stressful activities through stress management and potentially even reducing intense exercise in Type A personalities, but not always. The important concept to grasp here is to find balance between the two.