Currently I drink about 2 cups of coffee or tea a day (one in the morning and one in the afternoon usually). However I'm very sensitive to stimulants and drinking caffeine regularly keeps my resting heart rate really high, causes occasional heart palpitations, and sometimes trouble sleeping.

I've tried going without coffee, and while I can do it, I have trouble concentrating at work and even just enjoying my work. I'm borderline ADD (or possibly full on ADD, but haven't been checked). And I tend to lose focus easily if I don't have some coffee or tea in me.

For health reasons, I'd like to cut it out completely, but when I do my work performance seriously suffers.

I already work out (cardio and/or weight lifting) 5 - 6 days a week, and get an average of about 8 hours of sleep, but I still can't focus throughout the day without caffeine.

Are there any over the counter chemical or supplement alternatives for mental clarity that you've used with success don't cause the additional unwanted physical side effects that come with regular stimulants like caffeine?