Caffeine is well established to interrupt sleep. Interrupt doesn't necessarily mean a person wakes up either. It can mean 9 hours of less restful sleep than without caffeine. Over the long term, the less restful sleep can lead to other problems.
Coffee/caffeine doesn't stop me sleeping, it makes me not want to sleep.
The difference is, I can drink a couple of coffee (long black, strong) then go to bed and sleep soundly. But I have to argue with myself all the way to bed... because now I dont feel like I need it :-P
