Interesting! I am the exact opposite - I can drink coffee any time, and it doesn't affect my sleep at all. The reverse side of it it that it doesn't wake me up at all either. If I'm sleepy, giving me a cup of coffee is useless. I like the taste of it, but if I want to boost my alertness, I will need something else. I used to drink a lot of caffeinated drinks in my youth, so maybe I developed a tolerance - or maybe it's just my genes.