It isn't about caffeine, or coffee, etc. In order to feel the Spirit, one must be alert, mind, body, and spirit. having caffeine makes you act a different way then normal, impairing certain senses. Also coffee and tea, iced or not, shouldn't be consumed because of certain chemicals in them that harm the stomach.
I don't know if this is true but, I once heard that when exploring the Titanic they found leather pouches, hard and thick, turns out they were the stomachs of those on the Titanic and they turned that way from all the tea they drank.