Tea is actually a really healthy thing to drink compared to coffee or other "energy" beverages. Real tea leaves secrete multiple substances into water when you steep them in it, such as caffeine (although there is also decaffeinated tea), and catachins, which are antioxidants, and the amino acid theanine.
Catachins can be up to 30% of the weight of a tea leaf. In layman's terms, antioxidants are chemicals floating in your bloodstream that bond with some (bad) chemicals that are produced due to respiration, so that those chemicals do not bond with anything important, such as atoms/chemicals making up a cell (thus changing the makeup of the cell and being harmful). Breathing, while most of us think of it as keeping us alive, is actually a major cause of aging and cellular degredation... drinking tea will help to keep you alive longer
Caffeine is good for mental alertness, but it's also a pain in the ass. The half-life for biological metabolism of caffeine is 6 hours. So if you drink a single cup of coffee every day, you're taking in about 150mg of caffeine. 24 hours later, you still have about 18.75mg of caffeine in your bloodstream--to which you're adding another 150mg when you drink your coffee. Over time you build up a lot of caffeine, which can interfere with your sleep cycle (caffeine blocks the neurotransmitter adenosine that make you feel sleepy). Since you have so much caffeine in your body, your body makes more adenosine to help make you feel tired. If you suddenly miss your caffeine dose, the adenosine is in much greater supply than it needs to be and thus makes you crash hard. Best to stay off caffeinated stuff, only once or twice a week spread at least two days apart.
Theanine is an amino acid, and seems to have the effect of helping to temporarily relax the mind and reduce mental stress and it's effects. So, that's a good thing!
FYI: Black tea is more common in the US than green or white, but that's unforunate since black tea contains much, much fewer catachins than green or white.
Bottom line: Drink decaf green or white tea five times a day
