The main reason many of us drink any form of tea is for the caffeine content. Green tea is no different. While there are forms of green tea that are caffeine-free, most will still contain caffeine and that means they can offer all the same benefits.
Caffeine affects the brain in a great many ways, but among the biggest benefits are its ability to combat drowsiness. It does this by blocking the adenosine receptors in our brain and when you do that, you prevent adenosine from acting on neurons as it normally would. Adenosine is an inhibitory neurotransmitter (meaning it suppresses brain activity) and it is also a by-product that is produced by the brain using any form of energy-intensive function. This builds up during the day and makes us tired. But when you consume caffeine, you reverse this process and wake yourself back up!