The position in which the bedtime Shema and Hamapil is to be recited:

If one is fulfilling his Mitzvah of the nighttime Shema with this recital: Kerias Shema may be recited while sitting, standing or lying on one’s side. One may not recite Kerias Shema while lying flat on his back or front.[1] This applies even if one tilts slightly onto his side, until he is resting completely on his side.[2]

If one already said Shema after nightfall [in Maariv] or a woman: Some Poskim[3] rule that the above law only applies to a person who is currently fulfilling his obligation of the nighttime Shema with the bedtime Shema [such as one who Davened Maariv before nightfall[4]]. If, however, one already fulfilled his obligation of the nighttime Shema [such as he Davened Maariv after nightfall, or a woman who is not obligated in the nighttime Shema], and is simply reading it for the sake of protection, then it is permitted to be read in any position that one desires. However, other Poskim[5] conclude that nevertheless, initially, one is to recite it in a sitting position, with great concentration. Practically, according to Kabbalah it is to be read in a standing position[6], and so is the custom of Chassidim.[7]]

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[1] Michaber 63:1; Rama 239:1 “See chapter 63 regarding if it may be said in a lying position”

The reason: As in Shema one accepts the yoke of heaven, and doing so while lying is

[2] Admur 63:1

[3] M”A 239:5 based on Reb Zeira; M”B 239:6

[4] M”B ibid

[5] Chayeh Adam; P”M 239 A”A 5 question ruling of M”A ibid; M”B ibid; Kaf Hachaim 239:11

[6] Sefer Hakavanos Derush Halayla 5, brought in Beir Heiytiv 239:4; Mishnas Chassidim Miseches Hashechiva 2:1

[7] Rav Ginzberg in Seder Kerias Shema Sheal Hamita p. 6