With caffeine, for me it got to a point very quickly where I get dependent on it. - > What do you do in formal practice when you don't have caffeine? What do you do in your everyday life when you want to be fully aware in every moment but sleepiness kicks in?
When aiding with those substances, I find it difficult because I don't really face the sleepiness. It's a way of trying to escape it. But it's a part of experience, same as pain and everything else. Whether in concentration, do-nothing or inquiry, there IS a way to deal with sleepiness and even make it a useful tool to enter deeper states of relaxation. There are even monks (in Tibet I think) not sleeping for days on end to use this. Shinzen Young talked about this in one of his reatreats. Moreover, it's a good way to find out why one is sleepy in the first place (not enough sleep, nutrition, no exercise etc.)
That being said, it's still a legit way to sharpen your focus with tea of course, although I also think a side effect might be a faster monkey mind. I heard it often mentioned that caffeine (even in green tea) desensitizes your sensory perception over a prolonged time of use and I have to agree. I'm still in the habit of green tea but I really want to stop this soon or at least use it very rarely.
Just my thoughts, I'm not saying it's wrong or anything.