I have an exam soon, am still fasting, if i fail it will have nothing to do with fasting because i have the ability to pretty much go to the breaking time.
You just need to eat a lot, make your diet long-lasting slowly digesting carbohydrates, and ensure you are hydrated well.
Not eating for nineteen or twenty hours is unlikely to have any negative long-lasting effect on otherwise healthy individuals. As always, seek medical help or advise if you suspect it effects your health, because in the case it does harm your health in a bad way, i don't think you're allowed to fast.