Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter XII. Of the reason why while one sings the Psalms the rest sit down during the service; and of the zeal with which they afterwards prolong their vigils in their cells till daybreak.

Chapter XII.

Of the reason why while one sings the Psalms the rest sit down during the service; and of the zeal with which they afterwards prolong their vigils in their cells till daybreak.

This canonical system of twelve Psalms, of which we have spoken, they render easier by such bodily rest that when, after their custom, they celebrate these services, they all, except the one who stands up in the midst to recite the Psalms, sit in very low stalls and follow the voice of the singer with the utmost attention of heart. For they are so worn out with fasting and working all day and night that, unless they were helped by some such indulgence, they could not possibly get through this number standing up. For they allow no time to pass idly without the performance of some work, because not only do they strive with all earnestness to do with their hands those things which can be done in daylight, but also with anxious minds they examine into those sorts of work which not even the darkness of night can put a stop to, as they hold that they will gain a far deeper insight into subjects of spiritual contemplation with purity of heart, the more earnestly that they devote themselves to work and labour. And therefore they consider that a moderate allowance of canonical prayers was divinely arranged in order that for those who are very ardent in faith room might be left in which their never-tiring flow of virtue might spend itself, and notwithstanding no loathing arise in their wearied and weak bodies from too large a quantity. And so, when the offices of the canonical prayers have been duly finished, every one returns to his own cell (which he inhabits alone, or is allowed to share with only one other whom partnership in work or training in discipleship and learning has joined with him, or perhaps similarity of character has made his companion), and again they offer with greater earnestness the same service of prayer, as their special private sacrifice, as it were; nor do any of them give themselves up any further to rest and sleep till when the brightness of day comes on the labours of the day succeed the labours and meditations of the night.