GP-- okay, sure. I'll concede that point. In my experience it was not sedating at all, and if anything mildly stimulating. The variability in subjective experience, without any proper studies being cited, would point to a strictly psychological effect. For me, hot clear drinks (like tea or coffee) are generally stimulating, so I would associate a new hot clear drink with stimulation. If you've heard beforehand that rooibos is sedating, then you're more likely to have a slightly sedating experience. It's sort of like confirmation bias that way-- in the absence of a physiological effect, you experience what you expect to experience.
Also, glad to see you back!