You might be like me and apparently 1/5th of the human population doesn't really feel the effects of caffeine. Basically goes in and out of your system without 'proper processing'. Been years since I read about it, so things might have become more clear.

I can drink coffee/red bull all day and it'll do very little. Can drink just before going to bed and sleep just fine.

I'll try to find the study on this again if you're interested.