Question: Why do we recite the Shema again in the Bedtime Shema, after we have already recited it in the evening (Arvit) prayer?
Answer: Our Sages explain this for two reasons: (a) by virtue of the Shema and the verses attached to it, it protects a person from bad thoughts and destructive forces that come at night; (b) it is a mitzvah to recite the Shema, or words of Torah, right before sleep (and one even repeats them and continues until falling asleep). In addition, many people are accustomed to pray Arvit before nightfall, and they then rely on the Bedtime Shema to fulfill the mitzvah of Shema at its proper time.
Regarding the blessing of Hamapil, most communities say the blessing before the Shema, while the Chabad custom is to say “Hamapil” right before sleep, in order to minimize the interruption between the blessing and sleep. Some Sephardim say the Hamapil blessing without G-d’s name, since they may need to interrupt between it and sleep. Among them are those who refrain from saying it with G-d’s name if they go to bed after midnight.
If one needs to interrupt after the Shema by eating or speaking, it is permitted, and one should return and recite the Shema paragraph again. But after Hamapil one must be careful not to interrupt at all. However, one who has become obligated to recite the blessing “Asher Yatzar,” or who forgot a final blessing, must recite it then too.
The Bedtime Shema also has a spiritual aspect of taking account of the day that has passed. One then makes good resolutions: to strengthen good deeds and correct areas that need correction. According to Kabbalah, one even says the confession (Vidui) at that time (on days when Tachanun is said). Women too, in all communities, recite the text of the Bedtime Shema along with the Hamapil blessing.
Rav Yosef Ginzberg, Sichat HaShevua