There are two functional parts of the brain that play a key role in stress. These serve the functions of emotion and cognitive function. So I am calling them the 'emotional' brain (amygdala and its connections and medial forebrain structures including the medial prefrontal cortex) and the 'logical' brain (the dorsolateral prefrontal cortex, other parts of the prefrontal cortex, parts of the cingulate cortex and parts of the hippocampus).
Our logical brain plays an important role in dampening our stress levels. Parts of the logical brain become quieter and can even 'shrink' after prolonged exposure to stress. This is seen in melancholic depression. If stress reduces activity and makes parts of the logical brain shrink, then anything that promotes activity reverses the negative effects of stress. An amino acid known as 'Theanine' does just that. Theanine enhances activity within the dorsolateral prefrontal cortex. The logical brain is made more active, specifically the areas responsible for memory, higher thought and the allocation of attention. There is no need for a supplement - Theanine is found in tea. You just have to make sure you are eating the actual tea leaves to get enough of it, not just steeping the tea in water. One way of doing this is by drinking Japanese Matcha tea.
Tea contains both caffeine and Theanine which means you will feel both alert and calm at the same time. There are few other things out there which can make us alert and calm us down at the same time. Matcha does this extremely well. Enjoy your next cup!
Borgwardt S, Hammann F, Scheffler K, Kreuter M, Drewe J, Beglinger C. Neural effects of green tea extract on dorsolateral prefrontal cortex. Eur J Clin Nutr. 2012 Nov;66(11):1187-92.
Unno K, Iguchi K, Tanida N, Fujitani K, Takamori N, Yamamoto H, Ishii N, Nagano H, Nagashima T, Hara A, Shimoi K, Hoshino M. Ingestion of theanine, an amino acid in tea, suppresses psychosocial stress in mice. Exp Physiol. 2013 Jan;98(1):290-303.