Insomnia melatonin question

Hey guys I have a calculus exam tomorrow and my sleep disorder has gotten worse as the exams are approaching. My main issue now is that I stress myself out about having to fall asleep or else I will be tired during the exam. During the last few weeks I have taken melatonin to keep my sleeping pattern in line. At this point they don't have that much of an affect as they did when I first took them. I usually take 2mg and my questions is if tonight It would be a stupid thing to double the dose to 4mg (In a sense that would also work as a placebo on my stressed out mind). Would I risk being sleepy even after getting 8 hours of sleep? Or do you think it shouldn't be an issue?

Thanks so much on your advice.