A study has found that people who drank tea four times per day for six weeks, had lower blood cortisol levels (a stress marker) and were able to recover more quickly after a stressful event, compared to 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), January 2007.)