Well just today I was told that you should try studying in 20 minute periods, then take a rest for however long it takes for your mind to be ok again, then study for another 20 mins. You should try to get some sleep also, and avoid stimulants. Maybe you should take some time from your stiudy to try to relax. You know, relaxation and breathing techniques.

If you still cant get yourself to study, I think you should just go to bed, sleep as long as you can, get yourself a good breakfast tomorrow and try your best in the exam.