I've never checked my heart rate, but I do use caffeine on a regular basis; mainly before workouts and excercise. Usually it seems to kick in after probably 20 minutes and I go down in about two hours. This is just what I feel happen to me, not exactly what happens to my body, but in my head.

I actually hate caffeine because it is so addicting. You said you sip all day long, try going a day without any, you'll probably get a headache and just feel down all day. This is why I try to only use it before workouts. You get a much bigger effect from it as well, if you use it sparingly that is.