If you are drinking green tea in the morning or in mid-morning, maybe after 1-2 hours after breakfast, it can help you sharpen your attention as you can experience mild caffeine plus L-theanine. Green tea before or after a workout helps fat oxidation and reduces muscle damage that occured during exercise.
Drinking green tea in the evening can be helpful for some but there are few clear warnings. It is true that green tea has less caffeine compared to coffee but its caffeine can still delay sleep or reduce sleep quality if consumed too close to bed time. Experts recommend cutting caffeine 2-3 hours before going to bed.