I agree having a good diet and doing some form of exercise like soccer, running, walking etc. could be beneficial as it helps to keep the mind fresh and relaxed by reducing stress.
The other part though on waking up at 5am you don't need to do this extreme kind of routine to perform well in the HSC. Studying is not only about how many hours you are doing but the quality of it as well. If you are well rested then you can use that time you have more effectively as your brain is sharp. 4-6 hours of study a day consistently throughout the year and doing work on the weekends as well to get ahead is more than enough to succeed in the HSC.
In general this idea that waking up at odd hours like 4am or 5am will make you a success in life (school and then also later on in uni) is misplaced in my opinion. It may work for some people but then at the same time there will be people who wake up later and are able to achieve equally as good results. It's just a matter of preference and what is most effective for that person.
Also, if you wake up earlier that means you will sleep earlier, so is there really an extra 5 hours gained. A person who wakes up at 5am could study maybe till 7am and then they have to get ready for school, but they will have to sleep at 10pm or earlier (so you got 2 more hours of study). Someone who wakes up at 7-8am, could sleep at 11 or 12 (that's the 2 hours of study you gained waking up at 5am) so the time gained will end up being minimal.
I never woke up early for school still got a 99+ ATAR and in uni I would often wake up late and ranked top in my degree. What is more important than waking up early is consistent work throughout the year as opposed to cramming things in a few weeks before a test. If you do that however you get it done you will most likely succeed in getting a good mark.