I think its because your body gets used to it so you dont feel the effects so much. I wasn't aware of how much caffiene effected me until i stopped drinking it. I had a headache for about a week and now when I drink coke I feel it pretty much straight away. Guess everyone's tolerance is different. If I have a coke now I try not to have one after 3pm. That way I hope its out of my system by bedtime. It just makes sense, no stimulants before bed is my rule. Alcohol makes my dreams much worse too.