I honestly don't think it matters. I remember reading in Rafael Nadal's autobiography that he went to sleep at about 1:30am the night before he played (and won) the 2008 Wimbledon final, in what was the longest (and probably most grueling physically and mentally) in history.
For most of my SACs/Exams, my bedtime ranged from 11:15 to 12, but the night of my English Language exam I went just after 12 because I was nervous as hell, and lying in bed thinking wasn't going to help. It ended up being my most disappointing exam with pre-exam goals in mind, but not too bad. Additionally, the night of my Global Politics exam, I literally didn't go to bed until 1am. At that point, I was really emotional about the fact it was going to be my last school test, and again I didn't want to sit in bed thinking about it. In contrast, GloPol was my best performed exam, and saved my ATAR, which was ironic since it was by far my latest bedtime. For both, I ended up waking up at around 9:30 (GloPol start at 11:45 and EngLang 2:15). Keep in mind, for the rest of my SACs throughout the year during normal school times, I was fortunate enough to be able to wake up around 8-8:15 and get to school on time most of the time, so I was lucky there.
I'd suggest getting as close to 9 hours a night throughout the year, but disagree (anecdotally) with the proposition that sleep the night before matters a whole lot unless you have an early start. It's all mental from there, if you think that having a bad night will 'ruin' your chances of performing in the exam, chances are you've psyched yourself out from the start.