Addendum for recent item on two-part sleep.
For fun, tried looking for “first sleep” in Googlebooks. Only one definite mention, in a book on the Franciscan monks.
= = = = = START QUOTE:
In course of time S. Francis came to this place, and in the evening, after compline, he lay down to sleep as was his wont, that he might rise in the night to pray while the other brothers slept. Now this boy made up his mind diligently to watch the ways of S. Francis, that he might learn to know his holiness, and especially that he might discover what he did at night when he got up.
When Francis was at prayer in the hermitage of Greccio, in the last cell beyond the big cell, one night he awoke his companion who was sleeping next door to him from his first sleep, and the companion got up and went to the door of the cell where blessed Francis was, and the saint said to him : Brother, I have not been able to sleep to-night, nor stand upright to pray…”
Polistra's Mill 