It is permissible to recite Shema while standing, sitting, or lying down on one’s side. Indeed, according to Beit Shamai, one must recite the Shema of the evening while lying down, and the Shema of the morning while standing, as it says, “When you lie down and when you rise up.” Yet, the halachah follows Beit Hillel who interpreted the verse to mean the times that one is required to recite Shema – when people lie down (to go to sleep in the evening) and rise (wake up in the morning). According to Beit Hillel, whether the person stands, sits, or lies down is optional (Berachot 10a; Shulchan Aruch 63:1).
We can learn from this halachah that faith is not something detached from this world, something that can only be achieved under specific circumstances. Rather, the faith expressed in saying Shema encompasses all of a person’s life in this world, and therefore it is possible to recite Shema in any position.
In principle, it is permissible to recite Shema even while walking, as it says, “When you are walking on your way.” However, the Chachamim maintain that it is not proper for a person to accept upon himself the yoke of Heaven casually. Therefore, one who is walking should stand still when he recites the first verse of Shema (Shulchan Aruch 63:3; Mishnah Berurah 9). It is also forbidden to say Shema while lying on one’s stomach or back, since this is not a respectful recital (Shulchan Aruch 63:1; and see Shulchan Aruch, Even HaEzer 23:3).
Because of the importance of the first paragraph, in which we accept upon ourselves the yoke of Heaven, a person must be careful while saying it not to occupy himself with anything else, and not signal with his eyes, fingers, or lips (Shulchan Aruch 63:6).