Many people who advise you to "study early in the morning" are not correct and they don't really understand that each person is unique.
What is important to understand is that everyone is different. Everyone is unique. Some people like to study in the morning; others prefer to study in the night.
Do what ever you like and find naturally appealing and comfortable.
It also means that the saying "Early to bed and early to rise ..." is wrong. You do not have to study in the morning if you do not like it.
I myself liked to study late in night as compared to early in the morning, but my brother Dr Anil Bapna liked to study early in the morning.