Oh, you're one of those who can drink coffee at night without losing sleep? I'm envious.
For me it's a timing thing. On the weekends it doesn't matter and so I'll drink coffee as I please but during the week I have to get enough sleep to be able to function and so limit it after about 3pm. I guess it's partly psychological and partly the coffee as on the weekends I still find sleep. Maybe it's all about what's on my mind at any given point.
I think it is quite psychological that of coffee doesn't let sleep, most people who suffer from insomnia suffer it with coffee or without it. Coffee only exacerbates an existing problem. And in your case, if the weekend you find sleep without problems even if you drink coffee. Maybe you could be less strict in the week and have a coffee in the afternoon if it really provokes you.
I have trouble sleeping a lot, dreams and all, so I guess you're totally correct. I mean, sometimes I sleep really well but I have trouble falling asleep sometimes and also trouble with waking in the night and not being able to fall back to sleep...So I limit the caffeine situation. :)