Here is a reason why your studying should not exceed 4 hours
quote:
The Principle of Neuro-Transmitter Depletion Often students study or attempt to read for too long a period of time without stopping for a rest break. B.F. Skinner and other experts have concluded that the average student cannot usually study really difficult material efficiently for more than about four hours a day. Then efficiency and memory begin to suffer. Research shows that the average student cannot study effectively on the same subject for more than about four consecutive hours, even with short breaks every hour. What occurs is what is referred to as The Principle of Neuro-Transmitter Depletion. Neuro-scientists have developed techniques to monitor activity (usually defined as electrical impulses) and chemical changes in the brain during study or thought processing. If one studies the same subject too long, fatigue, boredom, sometimes slight disorientation may occur. It is a common result of too much consecutive study when even the most simple concept begins not to make sense any longer. The monitoring of brain activity and chemical changes indicate that studying too long results in a depletion of chemicals in the brain cells necessary for efficient processing of information. Therefore, for effective consolidation of material into memory storage, take frequent breaks (at least 10 minutes every hour) and do not attempt to deal with really difficult material for more than about four hours a day, and do not study any easier subject area (even with breaks) for more than four consecutive hours
I copied and pasted that from this site in case you didn't want to read the 'whole' article yourself. 
http://www.web-us.com/memory/memory_and_related_learning_prin.htm
Alex
