Why can't I do this anymore and still sleep at night?

When I was in my twenties we lived in an apartment. I made lots of friends there; one was a nice Greek girl. I would go upstairs to her apartment and drink coffee past dark. Not decaf, either but the real stuff. Then I’d go home and go right to sleep. Our husbands were taking night classes.
I don’t recall when it began; probably in my forties or earlier. But now I have to have a Melatonin before bed. I know it could be a whole lot worse but why can’t I drink caffeine before bed anymore?