As people have mentioned earlier, it's not unheard of to have weird sleeping patterns in the weeks leading up the the HSC. More than a few (including myself) had difficulty getting to sleep, and some (inc me) got about zero sleep the night before. But it's more trepidation than anything else - you've been hyped up about the HSC for years now. Only *now* that it's on your front doorstep is everyone freaking out going "omg, it's the HSC! I should be like, suicidal, and like, studying 23 hours a day!" etc etc. So effectively, I personally think a lot of people are just unconsciously psyching themselves out
Hopefully by the time it actually rolls around, you'll go on auto-pilot. The HSC really isn't as big as you think it'll be - when you're in it, a lot of people might think "eh, what a let down. It's just like any Yearly exam" (and it is!). When it's over, almost everyone will think "Oh, the HSC? Yeah, I remember that... sort of...I think..."
My only recommendation is that you give yourself 12 hours to fall asleep the night before your first exam. As you guys are still freaked out about what you haven't experienced yet, it'll take you a while to get to sleep.
In all seriousness, pretend it's just another yearly exam. You'll be in your school halls - have a lucky charm in your pocket if it will help you calm you down. Another thing - one of my psych friends suggested to me last year that to try and get rid of nerves, in the nerve-racking 5-10 minutes before the exam, plug in your ipod/earphones/whatever and listen to some music that you can "get lost in" - like your favourite song or something. Anything to take your mind off - remember, it's not the HSC, it's just the hype around it that freaks people out. So long as you don't let it get to you, you'll be fine, and if it has got to you already, remember you can always get out of it.
*hugs to all*