The best thing I can recommend right now is to take a break. Spend 15 - 30 minutes doing something to help relax yourself, as it is clear you are very stressed over the upcoming exam, and you are quite exhausted from the information they've thrown in your face already (I know how that goes; it is impossible to be able to retain everything they teach you over that long period).
Some things you could try to do to relax:
- Eat a small snack
- Turn on some soothing music. There are plenty of sleep / study / chill playlists on 8tracks.com
- Do something you enjoy doing and helps you relax. Watch videos on youtube, browse imgur, pick up the guitar and play a little. Anything to get your mind off of the work you're about to do.
- Envision yourself 24 hours from now after the test is over. Where will you be? Where do you want to be? Do you have what it takes to make it there?
Stay away from:
- Facebook, Twitter, Google+, any other social networking site. They will eat away at your break time and into your work time.
- Any video games. They may take up more time than you have to give.
For your neck: do some neck stretches, rolling your head around your shoulders, then take some advil or ibuprofen.
With 3.5 / 3.75 hours of studying left at your disposal, try to section off big ideas that you will be tested on. Mark down each area that you need to cover. Do this instead of starting at page 1 and reading until you fall asleep. After reading each section, summarize the ideas via writing them down. Good luck!