For the body that fasts cannot endure to sleep upon its pallet all the night through. Fasting naturally incites wakefulness unto God, not only during the day, but also at night. For the empty body of a faster is not greatly wearied by the battle against sleep. And even if his senses are weakened, his mind is wakeful unto God in prayer. It is better for a man to desist from his liturgy because of weakness due to fasting, than because of sloth due to eating.
—St. Isaac the Syrian