Difficulty in thinking properly during exams I am a student of science preparing for the JEE (India). When I normally practice problems, I find no difficulty in thinking straight. Solutions occur to me when I look at a problem, because for the most part, my fundamentals in Physics, Chemistry and Math are clear. 
But each time I sit down to take a test, I experience some sort of blockage in the mind, as if I'm not paying attention to the question (when actually I am paying attention). I find it harder to work my way around a problem during the exam than I would during practice. Had this difference between practice and exam been little, I probably would have ignored it. But it is so huge that I can feel how drastically it is affecting my score. It just feels like somebody has switched off a valve in my mind, drastically reducing my brainpower. And it's so frustrating to come home from a test and find easy questions that I could have solved, but didn't because their simple solutions didn't occur to me.
Is it because of the pressure that comes with a timer going on or could it be something else? 
And how do I deal with it?
I find that doing strenuous exercise, like an hour-long swim, before an exam relaxes me and calms the nerves.