Some stay up for a major portion of the night in prayer. Some will take this further by living with a regular reduction of sleep, sleep sitting or on the ground. The monks are going to renounce marriage and struggle with their sexual desires. This is particularly true of those who leave the civilized land to go into the desert or the Irish who wander from their homeland as exile on earth. Meaning of the action is multiple.
First possibility is purification of powers.
Understanding of the passions and appetites as clouding the operation of the mind and will. The spiritual faculties are the principal means of union with God. Thus a concentration of the energy and clearing of the powers. Notion of a link between body and spirit, so that the practices of the bodily discipline can free the emotions, the imagination, and ultimately the mind, from the effects of evil habits. Need to control the body precisely because of its influence on the operation of the mind.