Yeah, despite going to bed allowing for a 9 hour sleep before my exams, the inner voices always get you. It's important to try to go to sleep early, (look, even resting in bed is more beneficial than studying imo), but if you can't, there are worse things. It's not worth stressing about. Alon didn't sleep the night before the English exam and smashed it. There have also been uni exams when my quantity of sleep has been equivalent to 473ml of Red Bull, however many hours that may be. It isn't necessary to sleep really well to ace an exam - it's just going to make it a bit easier on yourself. Ultimattely, in that exam it'll be like "oh shit, this is it" that your focus will be keen enough you'll forget you feel tired.