Tea has long been hailed as a stress reliever, but this has seldom been tested scientifically. A recent human study has found that people who drank tea four times a day, for six weeks, had lower blood cortisol levels and were able to recover more quickly after a stressful event compared with those who drank a placebo beverage. (The effects of tea on psychophysiological stress responsivity and post-stress recovery: a randomised double-blind trial. Psychopharmacology (Berl). 2007 Jan;190(1):81-9).

Another study investigated the effect of theanine (an amino acid found in high proportion in tea leaves) on volunteers. They were given theanine, placebo or nothing, either at the start of a stressful mental arithmetic task, or midway through it. The results showed that theanine intake resulted in a significant reduction in the stress response (measured via heart rate (HR) and salivary immunoglobulin A (s-IgA)). Analyses of heart rate variability indicated that the reductions in HR and s-IgA were likely to be due to attenuation of sympathetic nervous activation. The authors concluded that oral theanine could mediate anti-stress effects via inhibition of cortical neuronal excitation in the brain. (L-Theanine reduces psychological and physiological stress responses. Biol Psychol. 2007 Jan;74(1):39-45).