At Johnath’s Blog there is an interesting article on the effects of Caffeine.
He makes the point that a stimulant like Caffeine is probably only useful if you are feeling tired. If you are already feeling tense and alert then Caffeine is likely to be counter-productive.
I have also read that the impact of caffeine varies between extroverts and introverts. Introverts are more likely to benefit from caffeine in the evening since they are more likely to be afternoon people, while extroverts are more likely to benefit from caffeine in the morning. Since people with Inattentive ADD tend to be introverts, they may benefit from using caffeine as an afternoon pick-me-up.
Many people with Inattentive ADD though have high anxiety levels, which are made worse by high levels of caffeine. This also means that they don’t tend to respond well to stimulant medication.
I usually only drink a couple of cups of tea in the morning and can’t sleep well if I drink caffeine in the evening.