In a particular week, you have a lot of schoolwork you want to get done. You decide to do a water fast for most of the week, to save time. You'll consume only water, not food.
A.) Are you allowed to do a water fast from Saturday night until Friday morning, even though this means you'll miss out on saying Havdala that week?
B.) Is this fine to do, even though you'll miss Havdalah? Or do the authorities advise against doing so?