Why is the daytime shema not treated the same way as the night sh'lema? At night the time for shema is the entire time that someone could be sleeping so why is the daytime shema not the entire time of the day that one would be awake or working until shkia?
Laser yisroel, Philadelphia, united States
A number of Rishonim ask your question.
a) The Sefer Yere'im (Rabeinu Eliezer of Metz) answers that Shechivah and Kimah differ from one another. The word "b'Shochbecha" is applicable to both the moment when one lies down as well as to the state of lying down over a period of time, unlike the word "uv'Kumecha" which refers only to the moment when one arises from sitting or lying down. Therefore, Keri'as Shema of the morning can be done only at the time when people are getting out of bed.
b) The Kesef Mishneh (end of the first Perek of Hilchos Keri'as Shema) answers that in fact Keri'as Shema of the morning can be read all day mid'Oraisa, but Chazal limited the Mitzvah to the first three hours of the day. They did so in order to promote Semichas Ge'ulah l'Tefilah (saying Keri'as Shema and its Berachos immediately prior to reciting the Shemoneh Esreh of Shacharis), and we hold that the Mitzvah of Tefilas Shacharis (Shemoneh Esreh) should be performed before the end of four hours.
See the Sefer ha'Chinuch (Mitzvah 420) who suggests a different answer.
Avraham Phillips