Caffeine is present in both tea and coffee, however, it has different effects on us.
First of all, the caffeine in tea should be called theine. It is recognised in 1838 as being identical to the caffeine in coffee, however, it affects our body differently because it forms different bonds with other substances.
When infused, theine bonds with tannins, which prevents it from being released rapidly, so it is absorbed over a longer period of time.
The caffeine in tea stimulates our central nervous system and the cardiovascular system by enlarging the diameter of the vessels in the cerebral cortex. This means tea sharpens our mind, increase concentration, eliminates fatigue and enhances our intellectual acuity. It is a stimulant.
Coffee, on the other hand, has a direct effect on blood circulation through the coronary system, stimulating an acceleration of the heart rate. It is an excitant.
Next time when mind power is needed, drink some tea. When physical boost is needed, drink coffee.