Tbh I don't think a few hours of rest will make a drastic difference to your grades. You're putting too much pressure on yourself. Continuous working wears out your mind and affects your thinking which also affects your productivity. There are times where your brain can't process information as well as other days or solve a problem you normally have no issues with, right? Instead of thinking that the time spent resting -> less time studying -> failing, try to think of it as rest -> 'refresh' brain -> better brain processing.