I mean, they do say to take your exams in similar conditions to how you prepared for it. I think my fear for you is that if you don't have caffeine, you'll crash. Jumping out of routine might do you more bad than you think.
You'd only have to hold it for 2.5 hrs max, and that's if you immediately wanted to go when question 1 pops up for bio (don't freak out... don't!).
aranjuez