I'm going to cover a couple of things here and I hope it helps those who need it:
First, you can get atleast 6 hours of sleep each night if you have your **** straight and its entirely possible to get even 7 hours of sleep. It all comes down to your habbits and time management. You have about a half hours worth of square away time before lights out and I would suggest you use that time wisely. From 2000 to 2100, unless you are cleaning weapons after some nasty training day, you will have time to square away your trash and get your hygiene done. If you want to have some extra sleep, I would suggest that you get your ass into the head, take a quick but good shower and get right back out. Its little things like that which go a long way. For those of you going back for seniors, know that things are a lot different from juniors. I personally found it to be slightly easier in some respects.
Right off the bat, you will notice that seniors is much more physically demanding. I found that it was mentally much easier. There is so much to accomplish during the 6 weeks, out of which you only train for about 4 weeks, that there simply isn't a lot of time for the staff to play games. However, that doesn't mean they won't FIND time to play games if it becomes necessary. Just be on top of your game because as seniors, you know what is expected of you. Also, the staff can differ substantially from one platoon to the next. My staff was very mellow compared to the staff of 1st platoon. There are some things which you simply can't control so the best thing to do is to just learn to live with it.
I cannot stress the importance of physical fitness enough. Going into seniors, my PFT was 260 and coming out it was at 290. The average PFT when I left for my platoon was near 270. So, don't get there out of shape and make sure you can run. The last thing you want to worry about while at OCS is your physical fitness.
Now, as far as the academics go, I wouldn't worry a whole lot about it. It's not rocket science and I never studied for it and left with a 91% academic GPA. If you are worried about passing the exams, just study up before the exam and look over the ELO's. You'll know what i'm talking about when you get there. When you are in class trying to keep yourself awake, take the time to find the answers to the ELO's, instead of sleeping! I used to just tear out the lessons right out of the knowledge before an exam and just study up during chow or whatnot. Remember, its all about time management and that doesn't mean sacraficing your sleep. Again, those who have already gone to juniors know what its all about and those who have yet to step across the tracks will soon learn.
I am not ashamed to admit that during juniors I questioned my choice to join the Marines. I thought about quitting and times were very tough at some points. However, I realize that it would have been the biggest mistake of my life if I had quit and i'm glad I pushed through it. Going back for seniors was probably one of the hardest decisions I have had to make in life so far but there is nothing like the pride you feel walking across the parade deck on graduation day. Take it from me, it is all worth the hard work and pain. I would suggest that you go to OCS with an open mind and don't sell yourself short. Everyone is capable of more than they believe. Be motivated, stay motivated and kick some ass. I hope this helps.