Urgh, please don't murder ye young bodies with lots of coffee and energy drinks.
A big bulk of the HSC(and approaching any major exams), and what I find is constantly understated is the mental approach of it. And I'm not talking about studying hardcore 24/7. Being confident, calm and well-rested is very important. A decent diet is preferable, but I find some exercise on the side good as well as it'll clear your mind and yay endorphins.
If you get into these sleep patterns, you'll find it hard to snap back to being mentally alert for 9AM/12PM starting exams.