i think part of the effect, when i feel relaxed after drinking tea, comes from when i can make it a ritual: ie. on a weekend morning, i'm in my bathrobe & pajamas, drinking tea & sitting on the couch reading something fun. when i drink it, bleary-eyed, in the morning @ work (especially when i'm drinking it in part b/c the office's heat isn't working again!!), it's not relaxing.
also, sometimes if i drink more black tea than i'm accustomed to (maybe a cup or two more) i get a bit jittery or my stomach hurts a bit (b/c of the caffeine, i assume).
i'm more inclined to believe that the ritual of drinking tea helps me, @ least, relax.