Gemara

The Kohanim holding limbs of the Tamid when halfway up the ramp, put the limbs down, and descended to say Shema.

Yoma 37b: Hilni ha'Malkah made a gold Nivreshes (lamp, or window pane). When the sun shone on it, rays came out, and everyone knew that the time for Shma came.

Question (Beraisa): One who read Shma with people of the Mishmar was not Yotzei, for they read too early.

(Abaye): The Nivreshes was a Siman for everyone else in Yerushalayim.

Berachos 8b (Beraisa - R. Shimon citing R. Akiva): A person can read Shma twice during the day, once before Netz (sunrise) and once after Netz, and fulfill his obligation for the night and morning Shma.

Question: "Twice during the day" implies that before Netz (but after dawn) is day. If he is Yotzei the night Shma, this shows that it is night!

Answer: Really, it is day. Since some people are sleeping then, it is still time for the night Shma.

(R. Yehoshua ben Levi): The Halachah follows R. Shimon.

9b (Mishnah) Question: What is the earliest time to say the morning Shma?

Answer #1: It is when it is light enough to distinguish between Techeles and white;

Answer #2 (R. Eliezer): It is when one can distinguish between Techeles and (the color of green) leeks, and lasts until Netz.

Answer #3 (Acherim): It is when one can recognize a friend four Amos away.

(Rav Huna): The Halachah follows Acherim.

(Abaye): The Halachah follows Acherim regarding the time for Tefilin. It follows Vatikin (pious people) regarding Shma:

(R. Yochanan): Vatikin are Gomer (finish) the morning Shma at Netz.

Support (Beraisa): Vatikin are Gomer Shma at Netz, in order to be Somech Ge'ulah to Tefilah (to begin Shemoneh Esreh right after Ga'al Yisrael, the blessing after Shma, which discusses redemption from Mitzrayim), and to pray during the day.

(R. Zeira): They learn from "Yira'ucha Im Shamesh."

22b (Mishnah): If one descended to immerse, if he can leave the water, get dressed and say Shma before Netz, he does so. If not, he should cover himself in the water and say Shma.

25a - Suggestion: Our anonymous Mishnah is like R. Eliezer, who says that one must say Shma before Netz.

Rejection: No, it can be like R. Yehoshua. It is like Vatikin.

26a (Mishnah): One may pray Shacharis until midday.

Contradiction (Beraisa): The time is at Netz, in order to mention Ge'ulah immediately before Shemoneh Esreh, and pray during the day.

Answer: That refers to Vatikin.

Rishonim

Rif (Berachos 4b): Rav Huna rules like Acherim.

R. Yonah (5a DH u'Matzinu): Abaye said that the Nivreshes was for others in Yerushalayim. I.e. it was not needed for the Kohanim in the Beis ha'Mikdash, for they rose earlier.

Rambam (Hilchos Keri'as Shma 1:11): The Mitzvah of daytime Keri'as Shma is to begin reading before Netz, in order that he will finish and say the last Berachah (after Shma) with Netz. This is about [a tenth of] an hour before the sun rises. If one delayed and read Shma after Netz, he was Yotzei. The Shi'ur is until three hours for one who transgressed and delayed.

Hagahos Maimoniyos (8): Also the Ri rules like Abaye, who rules like Acherim. Yoma 37b connotes that the time is after Netz. That is for the Tzibur, since not everyone can be precise to finish at Netz. This is unlike R. Tam, who rules that the time is only after Netz, like the Mishnah in Yoma. He says that Vatikin were unlike the Halachah. Also Abaye does not hold like them totally. He mentioned this time only to teach unlike Acherim, who say that the time is even earlier. Also R. Simchah rules like this. This is unlike R. Chananel, who says that even Vatikin did not say Shma until after Netz. My Rebbi, in the name of R. Pores, holds like the Ri; Avi ha'Ezri explained like R. Tam.

Question: In Yoma, Abaye connotes that the time for Shma is after Netz!

Answer (Rosh Berachos 1:10): The time (when they would see the rays) was for the masses who could not read as promptly as Vatikin.

Question: R. Yehoshua ben Levi rules like R. Shimon, who taught that one can be Yotzei the night Shma before Netz, and morning Shma after Netz. This shows that the time for morning Shma is after Netz!

Answer (Rosh): Even if R. Yehoshua ben Levi argues with Abaye, the Halachah follows Abaye, for he is Basra. Alternatively, R. Yehoshua ben Levi rules like R. Shimon's law that one can be Yotzei the night Shma before Netz, but not regarding the time for morning Shma. Alternatively, R. Shimon agrees that the time for morning Shma is before Netz. He did not say that one can read Shma twice before Netz and be Yotzei both the night Shma and the morning Shma, for he preferred to teach that one cannot be Yotzei the night Shma after Netz. Alternatively, if there was Ones, (on one day) he cannot read both night Shma and morning Shma in one time period (between dawn and Netz).

Rosh (ibid.): R. Chananel says that 'Gomer' means reading. He says that the time is from Netz until three hours, like R. Yehoshua ben Levi. Yira'ucha Im Shamesh refers to Keri'as Shma, i.e. Kabalas Ol Malchus Shamayim. This is wrong, for the Gemara says 'in order to be Somech Ge'ulah to Tefilah' and brings the verse. This connotes that the verse discusses Tefilah. Similarly, on 29b the same verse is used to teach that one should pray Minchah when the sun is about to set. "With the sun" can mean before or after. Also, 'Gomer' connotes (finishing,) unlike R. Chananel. The Rif explains like I said.

Rosh (Tamid 32b DH Shma): The Kohanim (who read when the appointee told them) were not Yotzei Shma, like it says in Yoma.

Rashi (Yoma 37b DH she'Hegi'a): The Nivreshes showed that the time for Shma came, like we say in Berachos that the Mitzvah is with Netz.

Ritva (37b DH ha'Korei): One who read Shma with people of the Mishmar was not Yotzei, i.e. he did not do the Mitzvah properly (but he was Yotzei). Also elsewhere it says he was not Yotzei (properly), e.g. 'if so, you were never Yotzei Sukah', and 'one who did not say these three things (at the Seder) was not Yotzei.'

Tosfos (37b DH Amar): We said (Berachos 25a) that the Mishnah of one who immersed can be even like R. Yehoshua; it is like Vatikin. This shows that 'Gomrim Im ha'Netz' means before Netz. The Mishnah discusses finishing before Netz, and we establish it like Vatikin. Here Abaye holds that the time is after Netz! Also, we read much after Netz. What do we rely on? Even though Rav Yehudah rules like R. Yehoshua, Abaye is Basra, and he rules like Vatikin! The Ri says that Vatikin agree that the Halachah follows R. Yehoshua. Letter of the law, the Halachah follows Acherim, and the time (begins from when one can recognize a friend, and) extends until three hours, just the best time is to finish before Netz and be Somech Ge'ulah to Tefilah at Netz.

Tosfos: We need not say that R. Shimon holds that the time for morning Shma is after Netz. (See the Rosh above.) Even if he would say so, we should not establish our Mishnah like him and unlike all the Tana'im (Berachos 9b, who explained the earliest time to be before Netz), for if so our Gemara should have explicitly established our Mishnah like him. R. Tam says that the time for Shma is after Netz, like our Mishnah and R. Shimon. Vatikin did unlike the Halachah, in order to be Somech Ge'ulah to Tefilah right after Netz. Even though Tefilos were enacted corresponding to the Temidim, and the morning Tamid was offered at dawn, before Netz, Zman Tefilah is at Netz. The Gemara (26a) shows that Tefilah is dearest at Netz. They said Keri'as Shma improperly for the sake of Tefilah. The Gemara knew this, therefore it did not ask.

Rebuttal (Tosfos): "Im ha'Netz" connotes that this is the primary Mitzvah. Also, it is difficult to say that the Halachah does not follow Abaye, who is Basra. R. Tam says that Abaye merely gave a Siman that the time for Shma is close to Netz, like Vatikin, but not totally like them. They say Shma before Netz, and we say it after Netz. He brought a proof from Megilah (20a) that any Mitzvah of the day applies after Netz.

Poskim

Shulchan Aruch (OC 58:1): The time for morning Shma begins is when one can recognize an acquaintance four Amos away. The ideal Mitzvah is to read it like Vatikin (Talmidim; Rashi - humble people who treasure the Mitzvos), who recited Shma a little before Netz ha'Chamah, in order to finish Keri'as Shma and its Berachos at Netz and pray Shemoneh Esre immediately afterwards at Netz.

Beis Yosef (DH v'Chen): Tosfos favors those who argue with R. Tam. R. Yonah, the Rosh, Rambam and most Poskim hold like this, so we follow them.

Kaf ha'Chayim (8): Shulchan Aruch ha'Rav says that if one cannot be precise k'Vatikin, he should read the first Parshah before Netz, even if he is not wearing Tzitzis and Tefilin, and later he will read Shma with its Berachos and be Somech Ge'ulah to Tefilah. This also fulfills R. Chananel's opinion. The Sugya in Yoma is like him, and several hold like him. The Rambam says that one who cannot be precise k'Vatikin should begin Keri'as Shma at Netz. Nesivos Shalom says that it is better to read Shma a little after Netz, for he can rely on R. Chananel, the Ba'al ha'Ma'or, R. Tam and possibly Rashi, but if he prays Shemoneh Esre before Netz, he has no one to rely on to do so l'Chatchilah.

Teshuvos v'Hanhagos (1:66): Chachamim promised a great reward to one who prays at Netz. Why aren't most people careful about this, especially in winter when Netz is almost 7:00 (and many Minyanim finish Ga'al Yisrael close to Netz in any case)? It is because many Gedolim, i.e. R. Tam, R. Chananel, Ba'al ha'Ma'or, Avi ha'Ezri and Rav Sadya Gaon say that one must say Shma after Netz. People do not want to enter this Safek. Mishkenos Yakov (77) says that one should be stringent for their opinion, and say Kri'as Shma after Netz. He says that also the Rif says so. I heard that ha'Gaon R. A.Y. Finkel Ztz"l would pray at Netz and repeat Shma after Netz.