Something suddenly struck me, and others might already have thought about it.
In the first Mishnah we are presented for the question “From when can we recite the Shma’ in the evening,” to which the answer is “from the time that the Kohanim enters to eat their Terumah.” The connection between the practice of reciting the Shma’ and the Temple-service is established here. That is why the mishnah a little later relates to the sacrifices of the fast and limbs, as well as the sacrifices eaten, is mentioned in this context, namely in order to make the context of the discussion of “until when” clear. The question of “until when” we can recite the Shma’ in the evening is therefore connected to how we relate to “until when” we can do the mentioned sacrifices as well, namely “until the dawn begin to rise.” But this is the Biblical commandment, the Rabbinic commandment has put a fence, “until midnight,” in order that the sacrifices and the eating of sacrifices may be done in due time.
Now, look at the answers we get to the unasked question of “until when may we recite the Shma’ in the evening.” Rabbi Eli’ezer says until the end of the first shift, the Sages until midnight, and Rabban Gamliel until the dawn begins to rise. We have three answers, but I am not so sure that we actually should read these three answers as being the same category. Look at the reasoning of them, the two first answers, those of R. Eli’ezer and the Sages, are dealing with the first part of the evening/night, that is, R. Eli’ezer says until the end of the first shift, which wouldn’t be far off from midnight, while the Sages state until midnight. What these two answers are dealing with is limiting the chance of pushing the fulfilling of the recital so far, that it would not be done in the end. The principle can be found in the first verse in the Pirqei Avot, “ועשו סיג לתורה” make a fence around the Torah. The principle is not to make new commandments, but to make sure that we stop before we get to the edge, sort of speaking, in order that we may not fall – To keep man from the sin. This both R. Eli’ezer and the Sages are agreeing about, but their discussion is where to put that “fence.” Rabban Gamliel on the other hand is not relating to this principle, for him that is obvious (and he is probably among the Sages, the majority here), he is rather pointing to the Biblical commandment, until the dawn begins to rise. He is saying that if you relate the time for when to begin the recital of the Shma’ in the evening to the Temple-service, then you should also do it when you try to decide until when it can be recited. That explains the following story about his sons, who knew that he agreed with the Sages, with the majority, but crossing the “fence,” then what? He relates that just as the Sages commands until midnight with the sacrifices, but the Bible until morning, so is it with the recital of the Shma’.
If this is the case, and it is – I have to admit – only me speculating, then he isn’t considering “when you lay down” as the whole time that people sleep, but rather basing his reasoning on the context of the Biblical commandment of the time of the sacrifices which can be sacrificed and eaten after the Kohanim enters to eat their Terumah (which can be eaten until the dawn begins to rise), and the Rabbinical commandments establishing a fence. He is relating the Shma’ to the Temple-service, not to when people are getting up or laying down.
For those of you who have greater insights, please let me know your thoughts on this.