If I drink caffeine in the evening, I don't necessarily have trouble sleeping but the problem is that I don't really feel like sleeping so I may be up until 1 or 2 before I realize I need to go to bed. Once I'm in bed, I'll fall asleep. But I have noticed if I drink coffee regularly I'll get caffeine withdrawal headaches when I skip a day. So now I drink decaf almost exclusively. And then I'll have an afternoon Coca-Cola pick me up after lunch to help get me back to a functioning level. So that means I'm getting about 34mg of caffeine/day which seems to be low enough not to cause the caffeine withdrawal headache.