The mitzvah of reading kri’as shema at night which we learn from the pasuk of “b’shachbecha” can be fulfilled at any time during the night (on a d’oraysa level), but when it comes to the mitzvah of reading shema during the day, which we learn from the pasuk of “uvkumecha”, the mitzvah can only be fulfilled until the third hour of the day. Why is there a difference between the two?
The Sefer haChinuch explains that Chazal understood “b’shachbecha” to mean any time that people go to sleep. There are a great many night owls who keep late hours and who sometimes retire closer to what most of us consider morning than night. However, rarer still is the person who sleeps in all day. Therefore, Chazal understood “uvkumecha” to mean only the morning hours, when most people wake up.
The Kesef Mishna in Hil Krias Shema raises the same question and disagrees with the Chinuch. He writes that min hatorah there is no difference between day and night – krias shema can be recited all day as well as all night. It is only a derabbanan mandate to finish k.s. of the day by the third hour. His proof: even if one reads krias shema after the proper time, the brachos krias shema can stil be recited. If min hatorah there is no kiyum mitzvah, how could one say brachos? (It seems implicit in his argument that brachos k”s are birchos hamitzvah, an interesting (and debatable) chiddush for another time.)
The Magen Avraham attacks the KS”M. If the obligation of shema applies all day and all night, then there is never a moment during which the mitzvah does not apply. Why then is krias shema considered a zman gerama mitzvah from which women are exempt?
The Sha’agas Arye (siman 12) addresses this question in the midst of a discussion of whether women are chayavos in the mitzvah of zechiras yetziyas Mitzrayim or not. The Sha’agas Arye at first contends that women are chayavos, as the mitzvah can be fulfilled during the entire day and then again during the entire night – there is no time that the mitzvah of zechira does not apply, so it is not zman gerama. However, the S.A. then backtracks. From the fact that women are exempt from reciting shema, which is the means by which we fulfill zechiras yetziyas Mitzrayim, it seems that they have no mitzvah of zechira (otherwise we would expect at least a Rabbinic enactment formalizing some recitation). But why are they exempt? The S.A. concludes that the mitzvah of zechira must in the end be zman gerama. Although the obligation to remember yetziyas Mitzrayim is continuous, in fact, the mitzvah really consists of two separate obligations which happen to coincide and come back to back with each other – an obligation of zechira during the day, which can only be fulfilled during the day, and an obligation of zechira at night, which can only be fulfilled at night. Since each independent obligation is limited in scope to a set period of time, although the gavra, the person, remains under a continuous obligation to remember yetziyas mitzrayim, these are considered independent zman gerama obligations.
The same logic applies to the mitzvah of reading shema. True, a person at any given moment may fulfill the mitzvah of reciting shema - either the shema of day or the shema of night - still, these are two seperate obligations and each one is limited in scope to being fulfilled at only one set time period.