BTW, there's also another angle to consider about coffee: how well you sleep afterwards. I recommend the book ‘Why We Sleep’ by Matthew Walker, it has plenty of info on why deep REM sleep is especially important. (Even though there's some criticism about the book, it still gives a lot of food for thought.)
I haven't seen definitive numbers on how long coffee works, but I'm pretty sure that it's more than the half-hour that Wikipedia claims, and Walker in particular says coffee in the afternoon does affect night's sleep. Personally for me, if I drink three cups in a day, or drink a cup or two for several days straight, then I can feel the effect on the following days—specifically in muscle tension and irritability.
I haven't seen definitive numbers on how long coffee works, but I'm pretty sure that it's more than the half-hour that Wikipedia claims, and Walker in particular says coffee in the afternoon does affect night's sleep. Personally for me, if I drink three cups in a day, or drink a cup or two for several days straight, then I can feel the effect on the following days—specifically in muscle tension and irritability.