Caffeine: Everyone knows that caffeine is an external stimulant and raises blood pressure, and is actually toxic in nature (it may be fatal for some animals). Caffeine is also addictive and can have adverse effects on the liver and internal organs over the course of many years. That being said, it is the component of tea which makes it energizing and refreshing, which is why people are willing to risk it.

can increase energy and mental focus. Green tea contains a small amount of caffeine, which a 2008 study in Nutrition Bulletin found can improve mood, cognitive function and physical performance. Green tea contains less caffeine than does coffee, and provides L-theanine, an amino acid shown to promote a state of calm awareness. Result: green tea provides the benefits of alertness associated with caffeine without the “jittery” feeling often experienced as a side effect of coffee.

When I was conducting research into South Africa’s native herbs, I was served rooibos everywhere I went. Red in the cup, fragrant to the nose and pleasing to the palate, rooibos soon became a favorite. In every meeting, I looked forward to that red tea, which is one of the only native herbs of South Africa to achieve significant commercial status.  My wife and I keep it in the tea cupboard at home, and we often turn to rooibos when we want a cup of tea without caffeine.
Several large, population-based studies show that people who regularly drink black or green tea may be less likely to have heart attacks and strokes. However, people who drink tea tend to be different from people who don't drink tea. "We can't quite disentangle whether it's their tea drinking or something else those people are doing that lowers their risk of cardiovascular disease," explains Dr. Sesso. "Some experts believe that tea may have cardiovascular benefits, but it's not considered a slam-dunk proposition."
Grown under shade for three weeks prior to plucking, gyokuro is one of the most exclusive varieties of tea produced in Japan.[60] The shading technique imparts a sweeter flavor, and produces a particularly rich color thanks to the higher amounts of chlorophyll in the shaded leaf. Gyokuro tea is associated with the Uji region, the first tea-growing region in Japan. It is often made using smaller-leaf cultivars of the tea plant.[61]