Hey man,
I'm a Senior in Aerospace Engineering and I've had to drop a couple classes this semester. I was averaging 0-4 hours of sleep for over 2 years and ended up working up a ridiculous tolerance-- inducing a vicious cycle. During peak dosing (my brain was getting blasted), I was struggling to just turn in homeworks and no longer was I acing my exams. Haven't quit 100% because I keep slipping every now and again but I'm determined.
Like Calo, I have also experienced a significant degeneration of my intellectual capabilities. I used to be praised for having a very good memory, but now it's pretty pathetic. I used to be able to deconstruct complex math/engineering problems with elegance but now I can barely identify connections. My writing/reading levels have dropped, and many times I have to read simple sentences over before it registers. I'm just not sharp and it really pains me because I have 5 classes left to graduate and I'm tanking my GPA or dropping classes.
Fortunately, my older brother who was also addicted for about 3 years, gives me hope. He's a true genius and has helped me cope through the times. He felt these symptoms of withdrawal for a long time but now he says he's regained his cognition and his mind is much sharper-- much healthier, and his intellect has excelled much further than it ever was on addy. The work-ethic comes, and you'll realize the quality of your study is much better. The material marinates much better, much more naturally, and you will perform better.