I recently ran across this excellent description of what happens with your autonomic nervous system and how it affects your body and brain under chronic stress from the Wisebrain website. The website is a nonprofit effort of Rick Hanson and Richard Mendius, the authors of The Buddhas’s Brain, which is an excellent book that describes what happens to your brain and body when you meditate regularly.
If I ever needed a reason to meditate, stay calm, and cultivate a positive attitude, I got it from his description. I was really struck by how the brain changes when it thinks we are under threat and becomes more likely to perceive threat and more sensitive so it responds quicker – a vicious cycle! I also got a reminder and a good picture of how our bodies aren’t built to withstand long term moderate activation of the stress response (aka: “fight, flight, or freeze”) most of us typically are subject to.
Hanson points out that we can manipulate both the gas, and the brakes on our nervous system, and offers several ways to activate the brakes, including meditation. He makes a case for focusing on internal changes versus being overly focused on trying to change external conditions.
Hanson has lots of other cool resources on his website, and you might want to take a look at the other Train Your Brain resources on the list where this one came from.