Green tea is not only a refreshing beverage, it is also considered medicinal because of its wide array of health benefits. It can help prevent diabetes and cancers, reduce acne and can also be used for hair benefits. It also helps to boost metabolism which directly contributes in losing weight and preventing obesity.
Metabolism is the amount of energy a human body uses, or the amount of calories your body burns in a day whether you are resting or working.
Drinking green tea is considered as one of the easiest approaches to boost your metabolism. A study published in the American Journal of Clinical Nutrition reported that green tea increases the daily energy expenditure by 4%. 
Green tea contains caffeine
It is because of its caffeine content that green tea is helpful in boosting metabolism.
An 8 fl. oz. cup of green tea contains around 25 mg of caffeine. Caffeine is a stimulant and the world’s most popular psychoactive drug. It stimulates the central nervous system and most notably blocks the adenosine response which prevents you from getting drowsy.
Caffeine in green tea boosts metabolism
Caffeine can significantly increase your metabolic rate. It can also increase your Resting Metabolic Rate (RMR), which is the amount of energy used up by your body when you are resting. A research conducted by AG Dulloo et al. on eighteen college students concluded that caffeine can increase RMR by 3-4% in two and a half hours. 
- green tea extract increased 24-hour energy expenditure by 4%. 
- caffeinated drinks can increase body metabolism in account of thermic effect. 
- caffeine and catechins, which are abundant in green teas, can significantly increase daily energy expenditure. Catechin-caffeine mixture increased it by 4.7% and caffeine only increased it by 4.8%. 
- the mean increase in metabolism was found to be 7 (+/-4)% during three hours after caffeine consumption. 
But don't drink too much
Moderate doses of green tea can help you feel relaxed and alert. But consuming green tea in higher doses can have some adverse effects in account of caffeine and tannins found in green tea.
If you consume too much caffeine, it can lead to sleep deprivation. When your sleep patterns are disrupted, it can lead to other problems like tremors, mood depression, anxiety, nervousness etc.
Caffeine overdose can cause irritability, hyperglycemia, nausea, dizziness, upset stomach and cold sweats. 
If you want to avoid an upset stomach, then refrain from drinking green tea on an empty stomach because the tannins in the green tea can increase acidity in the stomach.