Learn about nootropics and you won't ever need those energy drinks.
Your body and mind is tired because of a lack of nutrition since much of the food today is nutrionally dead.
All you need is barley/wheat grass juice powder, cacao and organic eggs to give your body more fuel then it needs.
Drink the Barley/wheat grass juice in the morning and study, when tiredness kicks which happens after 2/hours have coffee mixed with cacao which will fuel you for several hours more.
When tiredness kicks in again (noon-time) go for a 30 min run outside or even better do a 30 min HITT session, this will fuel you for several hours more and as you dip again have another coffee.
You can easily get 8-10 productive hours like this and if you ever feel jitteriness due to caffeine overload, have tea because tea contains L-Theanine which counteracts caffeine and calms you down.
A lot of people I know take caffeine with L-Theanine in pill format but I like things natural, have your tea late in the afternoon, you will sleep better this way too
There is a reason why all our odayaal's are drinking so much tea, because L-Theanine distresses your mind and puts you at ease.