Deep relaxation and meditation have long been known to help with anxiety disorders and major depression. Because both anxiety disorders and major depression reflect an imbalance in brain chemistry, it would seem that relaxation techniques can influence brain chemistry. How do they do that?
The sympathetic response to stress and fear consists in a rapid release of stress chemicals into the bloodstream, including cortisol, adrenaline, and noradrenaline. The reaction to fear is almost instantaneous, but shutting down the stress system takes more time. For people with anxiety disorders, the stress system never shuts down.