A cup of green tea contains between 15 and 50 mg of caffeine. Certain cognitive benefits are associated with caffeine consumption, such as a reduction in the likelihood of Parkinson's disease and a temporary increase in short term memory. Further, caffeine consumption has been linked with greater athletic performance, healthy weight loss, reduction in duration and severity of headaches and is effective in treating the symptoms of asthma.
Effects on obstructive sleep apnea-related brain deficits
University of Louisville researchers report that green tea polyphenols may stave off the cognitive deficits that occur with obstructive sleep apnea, in the second issue for May, 2008 of the American Thoracic Society’s American Journal of Respiratory and Critical Care Medicine. Researchers examined the effects of green tea polyphenols administered through drinking water, on rats that were intermittently deprived of oxygen during 12-hour “night” cycles, mimicking the intermittent hypoxia that humans with OSA experience.
Effects on bacterial and fungal infections
One study shows that green tea reduced the severity of rheumatoid arthritis in rats; however another study shows that tea increases the risk for rheumatoid arthritis by 78% for heavy drinkers and by 40% for occasional drinkers.
Weight loss and cholesterol
Green tea and its extract have been shown to fight obesity and lower LDL "bad" cholesterol—two risk factors for heart disease and diabetes. One study in the Netherlands and a study in Japan showed that green tea did both. In the Dutch study, participants who drank caffeinated green tea lost more weight, but even those who typically drank the decaf variety saw a decrease in their waistlines and body weight. Researchers speculated that the caffeine helps with fat oxidation. In the Japanese study, 240 men and women were given varying amounts of green tea extract for three months. Those who got the highest amount lost fat and weight and had lower blood pressure and lower LDL "bad" cholesterol.
Reference : http://en.wikipedia.org/wiki/Health_effects_of_tea