I would say it depends on what kind of exam you are taking. Some exams (I know you were talking about insurance licensing, etc the other day) can be crammed for, IF that is your style. On other exams which require thought processing beyond regurgitation you will likely need to be more rested. Is it a multiple choice test where you are just expected to have the info in your head? Or is it a test where the answers seem to be obvious but you need to find the one piece of info in the question that is meant to trip you up? That type of test usually means delving a bit deeper into the info, studying the exceptions rather than the rules.
It depends on how dire the exam is, what time of day you are taking exam, how you test period. I have found it more efficient to tailor my study skills to the specific test.