TL;DR: Green tea is good for you and you can have it several times a day i.e. there is no “one” good time to drink it.
You can drink green tea at any time of the day or night. Tea in general contains less caffeine than coffee and other drinks such as Coke, Redbull, etc. and should not impact your sleep much (unless you are highly sensitive to caffeine). Green tea contains relatively lower amounts of caffeine even in comparison to other teas (for e.g. black teas) and is considered beneficial to health due to the presence of antioxidants and other minerals in it.
I also highly recommend using loose leaf tea to brew your tea if you plan to consume green tea regularly. Teabags almost always (95%) lack the depth of flavour and nutrients present in loose leaf tea.If you’re looking for one of the finest green teas I have ever had, check out The Jade of Bengal | Single-Estate Darjeeling Green Tea. You are guaranteed to love this autumn flush Darjeeling green tea.