תּוֹרָה קיז

At the conclusion of Shabbos [motzaay Shabbos] — the revelation of Eliyahu begins. And through this the falsehood — which is from the aspect of the serpent that brought death to the world — is repelled.

And therefore it is then difficult to sleep. For sleep is also from this aspect (see the main text).

Also because the "havdalah" of Motzaay Shabbos is the aspect of da'as — for: ""if there is no da'as — where is havdalah?"" [Jerusalem Talmud, Berachos ch. 5]. And therefore it is difficult to sleep — for sleep is the departure of da'as.