I have felt the remission you describe, though it's been a long time. I, like you, prefer to stick to a normal sleep schedule. However, when I was much younger (around age 12) I would spend hours on end drawing - usually at night. I would break past the physical discomfort to keep going until daybreak sometimes. You used the term "functional zombie", which I think is a very accurate way to describe that state. Good luck with your exam!