While I also have normal blood pressure on the low side and dodge the occasional faint, the lethargy and sleep side I always consider separately. I have a low tolerance for lack of sleep and when my body decides it is time to go to sleep there is no fighting it. I always liken it to a little man turning off all the lights. I am lucky if I can brush my teeth before I go under. Also my body temp always drops like a stone when I am tired and I often start shivering uncontrollably until I get into bed. This sounds similar to what you are experiencing. I am always fine after I have slept so I would suggest finding a way to feed more sleep sessions into your study schedule even if they are short and make sure you get back on a better sleep schedule once this cram session is over.