Well, I think that either is effective as long as you get enough sleep before an exam. However I'm being hypocritical as today is the day of my english paper 2 trial and last night i stayed up until 2 practicing and woke up at 530 to practice more. When I tried to get to sleep though I found it really hard because my head was buzzing from all the stuff I had been doing, so you definitely need time to wind down. However, even though I am functioning on 3 and a half hours sleep I am feeling pretty confident for the exam... so test out both and see what your body can handle!