TT #48 · Parashas Va’eschanan · Passuk 6:4
Hear, O Israel: Hashem is our God, Hashem is One.
Torah Temimah points to Berachos 13b (R’ Ila in the name of Rav: one who said the first verse and then fell asleep has fulfilled the obligation).
English: R’ Ila said in the name of Rav: If one said “Hear, O Israel, Hashem is our God, Hashem is One” and then was overcome by sleep — he has fulfilled his obligation.
Custom — the first verse alone constitutes the core obligation when further recitation is prevented by sleep.
One who manages to recite only the first verse of Shema before being overcome by sleep has still fulfilled the essential obligation of kabbalas ol malchus shamayim.