There's an interesting side to caffeine addiction. When you've reached a certain point of addiction, it's not the caffeine that hampers you sleep but the lack of it. There are no scientific research or statistically significant data available on this, but I myself was a caffeine addict (proudly so hehe), and I have experienced this firsthand!
Those days are long gone now, and I can keep my caffeine in check now :D
Well, there was a time in college when I needed coffee or I wouldn't sleep. Those days are long gone, too. One thing I know, I never drink string coffee in the evening.