As you say, the variation is wide. I can drink tea up until about 6pm without any problems, but if I have even one coffee after lunchtime I'm going to be up half the night regretting it.
Depending on the type of tea you drink, this could be due to the calming effects of l-theanine [1] in tea.
I have the genes that extend caffeine half-life. If I drink an American large coffee, I get anxious. I can drink a whole pot or two of green tea and feel a nice lift in mood.
As others have mentioned, any coffee after noon and I can't fall asleep until around 2am. I avoid caffeine most days except for the occasional morning kombucha.
[1] https://www.reddit.com/r/nootropics/wiki/faq#wiki_l-theanine
Re:Caffeine use disorder
by maldusiecle on Monday March 14, 2016 @03:31PM
(#11284958)
It's the opposite with me. I can have coffee in the late afternoon, in some cases even in the evening, without problems--but if I have a tea after 3 pm or so, I'm sure to have trouble.
Might have something to do with the strength of the tea, though.
As an aside, the whole "8 hour half-life" is astonishingly high, to me. Caffeine is nice, but it doesn't seem to have that strong an effect on me, and certainly nothing I notice 5-8 hours after. I drink quite a lot of caffeine these days, but even at times when I went off caffeine for a few weeks, I didn't get effect durations any longer than 1-3 hours.