That explains a lot. I really can't relate do other coffee drinkers describing their experience. I often drink 0.4 liter of black coffee to get rid of the late evening sleepyness when I want some hours of productive free time for gaming or programming before bed. When I try to sleep ~3h later the effects of the coffee are largely gone and I fall to sleep within 5 minutes. I guess I've got a lot of that enzyme. ;) (which is nice)