Commentary Purg XVIII 143

The verb Dante uses to describe his floating state of consciousness, vaneggiai (rambled), picks up an earlier phrasing, when he compares himself to one who sonnolento vana (rambles in his drowsy mind -- verse 87) after Virgil has finished his explanation of love and free will.  There he is falling into a fatigue that mirrors the sin purged on this terrace.  Here he finally gives in to that weight of somnolence.