Berachot 9a, 4b

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{Berachot 9a}

 Rabbi Zera said: so long as he does not say Hashkiveinu - {"who causes us to lie down" - since even though it is the night Shema, it is not the beginning time of lying down but rather the end time of lying down. - Rashi}

And all of this is halacha.

However, this that Rabban Gamliel said {that you can say the entire night} and this that R Shimon ben Yochai said {that the night one you can fulfill right before either dawn or sunrise} is bedieved - after the fact - and even if you did so willfully {bemeizid} and read keriat Shema before dawn you will have fulfilled your obligation, even though you are not permitted to do this, for we learnt in a brayta {Berachot 4b}:

The Sages made a fence to their words in order that a man should not come from the field in the evening and say 'I will eat a bit, drink a bit, and sleep a bit, and afterwards I will read Shema and pray' and if sleep snatches him he will have slept the entire night; but rather a man should come from the field and go to the house of gathering {shul} or the house of study - if he is used to reading {Scripture} he should read, and if he is used to learning he should learn - and afterwards read the Shema and pray. And all who violate the words of the Sages is liable the death penalty.

 and this that Rabbi Shimon {ben Yochai} said that there are times that a man reads the Shema before sunrise is only in a situation of duress, such as where one is drunk or ill. And if he does this deliberately or through negligence he does not fulfill his obligation.

{Brachot 9a} When R Yitzchak beRabbi Yosef came {from Eretz Yisrael}, he said this statement of Rabbi Yehoshua ben Levi was not said directly by him {Rabbi Yehoshua ben Levi} but was deduced by inference {by R Acha bar Chanina}. For a couple of scholars became drunk at the wedding feast of Rabbi Yehoshua ben Levi and they did not say kriat Shema. They came before R Yehoshua ben Levi. He said to them: Rabbi Shimon ben Yochai is worthy to be relied upon in time of duress.

And so too the words of R Shimon ben Yochai who says there are times that a person reads Shema of morning immediately after dawn {and thus before sunrise} isbedieved - after the fact - or else in time of duress, such as one who must arise early to go on the road, but lichatchila - initially - no.

{Brachot 4b} The Master {that is, the brayta} said: a man reads keriat Shema and prays. This supports Rabbi Yochanan who says: Who is a member of the World to Come? He who juxtaposes geula {the first blessing after Shema} of evening to the prayer of evening.

And Rabbi Yehoshua ben Levi says the prayers {Amida/Shemoneh Esrei} were instituted in the middle, that in the morning he reads the Shema and prays, and at Mincha time he prays the Mincha prayer, and in the evening he prays the prayer of evening, and afterwards he reads the Shema of evening. It comes out that the reading of the Shema of morning is in the beginning of the day, and the reading of the Shema in the evening is at the beginning of the night, and the prayers are all in the middle.

And we do not hold like Rabbi Yehoshua ben Levi, and we establish {the halacha} like Rabbi Yochanan, for he has a brayta that supports him.

And even though he says hashkiveinu {the second blessing after Shema at night} between geula and the prayer {Shemoneh Esrei}, it does not interrupt, for since the Sages established hashkiveinu after geula, it is like a lengthened geula [blessing].

And in the morning prayer {Shemoneh Esrei} one also needs to juxtapose geula to the {Amida} prayer. And even though in the beginning one says (Tehillim 51:17)

יז ה, שְׂפָתַי תִּפְתָּח; וּפִי, יַגִּיד תְּהִלָּתֶךָ.

17 O Lord, open Thou my lips; and my mouth shall declare Thy praise.

since the Sages established ה שְׂפָתַי תִּפְתָּח in the prayer, it is like a lengthy prayer {Amida}.

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Rabbi Yehoshua ben Levi said: even though a person said keriat Shema in shul, it is a mitzvah to read it upon his bed, and specifically the first parsha of Shema, as we say in Perek HaRoeh {Brachot daf 60}.

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