[acjama]

There is a difference whether you simmer the tea for several minutes like one does with black tea, or the recommended 45 seconds (or so). If the tea leaves are dried, 45 second simmering will only bring a mild taste, but that's just because the tea is old. Basically, green tea is supposed to be consumed while fresh and let simmer only a short time.

Also as slightly related, there are some flu medicines that have caffeine to give the body strength to rest (for when the body is exhausted beyond being able to rest properly). The relaxing effect of the green tea might be attributed to that. Anyway, I don't connect green tea with relaxation at all. Quite the opposite.

With regards to headaches, morally I can only say that if you see a connection between green tea and them, I'd suggest you find an alternative.