I have a weird and unstable relationship with caffiene. I usually have 1 cup of coffee in the morning with cream; sometimes I'll have a second cup (or plain green tea or black tea with milk) around 2/3pm. Some days it doesn't affect me in the slightest, but other days, when I'm stressed or it's made stronger than I'm used to, it can give me a sub-anxious feeling and then I can't focus at all.
But then sometimes, on a regular day, if the coffee is (supposedly) stronger, it puts me to sleep. Turkish coffee works this way on me.
I don't have an explanation