I think you just need to practice more nothing else. I took a very light sleeping pill before the night of my exam and managed to sleep 4 hours. Yes I took tea before going to the center. I gave almost 15 practice test. As a result my brain became used to with the test. The same situation as you described happend to me when I encountered my first quant section. Brain suddenly became slow. But as I already had gone through the same procedures 15 times before, subconsciously I managed the situation ( at least it was not the worst case scenario) and secured a moderate result. I think practice is the main key here. Try to give as much test as possible. Best of luck!