Coffee and Tea
In my life I've experimented with my caffeine intake. Sleep has always been tricky for me and caffeine is a big part of that. When I was still in college, I went through a phase of constant coffee drinking that resulted in a year of non-stop stomach problems. About a year ago, I was drinking no caffeine at all. I don't know what my ideal is, but caffeine has positive affects on productivity in general, so completely cutting it isn't great, and stomach problems are for sure not good.
I don't have the answer and I'm not the best optimizer, but currently, I drink a single cup of green tea a day. Today, I'm drinking a cup of coffee. I had to be up early and didn't go to bed soon enough last night. The coffee has more caffeine and feels like a treat. I've got to be careful though, since coffee and it's higher caffeine (and subsequent sleepless nights) is a very seductive thing. Funny how the stuff that is a treat can so quickly become the norm.