The Conferences Of John Cassian by John Cassian
Chapter XXXV. A question, why sometimes when we are fasting more strictly than usual, we are troubled by carnal desires more keenly than usual.
Germanus: You have very clearly explained a most difficult question, and one which, as we think, is unknown to many. Wherefore we pray you to add this also for our good, and carefully to expound why sometimes when we are fasting more strictly than usual, and are exhausted and worn out, severer bodily struggles are excited. For often on waking from sleep, when we have discovered that we have been defiled we are so dejected in heart that we do not even venture faithfully to rise even for prayer.