A tricky and a good question. Research have found that the more you sleep good and the more you dream, the better your memory is. There is a particular phase of sleep called the REM sleep during which whatever is learnt or memorized is consolidated and that helps in good recall later. So my advice will be to have a good sleep throughout the night and you can wake up early morning to study whatever is left. With this you can have a good recall the next morning. Further, not sleeping at night causes anxiety symptoms the next morning that can hinder effective recall of academic material. Hence always have a good 6 hours sleep the day before your exam and at other times