The text states that Alexis <em>"has always been a night person"</em>. If she is a night person, she finds it easier to get things done during the night, which explains why she would choose to study late at night. She is certainly using her personal preferences, and also the time that she is most functional, to form her study routine.
Compound there are two sentences connected by a semicolon, you can tell because they each have their own subject (the musician, she) and their own verb (preform, had).
