This steamed green tea contains caffeine, which is naturally present in tea leaves and gives the infusion a more noticeable lifting effect than most herbal beverages. The actual caffeine level depends on brewing time and temperature, but generally steamed green teas fall into the moderate range - typically less than black tea but more than white tea.
For timing, many people choose to drink it with breakfast or as an afternoon pick-me-up during work or study. If you are sensitive to caffeine, it may be better to avoid a strong infusion late in the evening. Very hot tea can also seem sharp on an empty stomach for some, so a slightly cooler brew or having it alongside food may be more comfortable.