I haven't seen this discussed in a while, so I thought I'd revive it in case there is any new info out there (and because it's really been affecting me lately). After drinking several infusions of Silver Needle (Yin Zhen), I got a headache and some sinus pressure. This pressure is similar to how I feel after drinking alcohol, so I assumed it was due to dilated blood vessels and nasal/sinus tissues. Yesterday I drank several infusions of Jade Flower (tea ball from Tao of Tea that has a wonderful flavor, not just the novelty of the tea ball) and got the same thing, including a headache near my temples which pretty much ruined the rest of my day. I haven't noticed this with black (fully oxidized) teas, and am in fact drinking several cups today to verify this.
I don't know much about the chemistry of tea, but I presume these headaches are caused by some component of tea which is chemically altered or destroyed during oxidation.
Thanks!
Alan