(א) מֵאֵימָתַי קוֹרִין אֶת שְׁמַע בְּעַרְבִית. מִשָּׁעָה שֶׁהַכֹּהֲנִים נִכְנָסִים לֶאֱכֹל בִּתְרוּמָתָן, עַד סוֹף הָאַשְׁמוּרָה הָרִאשׁוֹנָה, דִּבְרֵי רַבִּי אֱלִיעֶזֶר. וַחֲכָמִים אוֹמְרִים, עַד חֲצוֹת. רַבָּן גַּמְלִיאֵל אוֹמֵר, עַד שֶׁיַּעֲלֶה עַמּוּד הַשָּׁחַר. מַעֲשֶׂה שֶׁבָּאוּ בָנָיו מִבֵּית הַמִּשְׁתֶּה, אָמְרוּ לוֹ, לֹא קָרִינוּ אֶת שְׁמַע. אָמַר לָהֶם, אִם לֹא עָלָה עַמּוּד הַשַּׁחַר, חַיָּבִין אַתֶּם לִקְרוֹת. וְלֹא זוֹ בִּלְבַד, אֶלָּא כָּל מַה שֶּׁאָמְרוּ חֲכָמִים עַד חֲצוֹת, מִצְוָתָן עַד שֶׁיַּעֲלֶה עַמּוּד הַשָּׁחַר. הֶקְטֵר חֲלָבִים וְאֵבָרִים, מִצְוָתָן עַד שֶׁיַּעֲלֶה עַמּוּד הַשָּׁחַר. וְכָל הַנֶּאֱכָלִים לְיוֹם אֶחָד, מִצְוָתָן עַד שֶׁיַּעֲלֶה עַמּוּד הַשָּׁחַר. אִם כֵּן, לָמָּה אָמְרוּ חֲכָמִים עַד חֲצוֹת, כְּדֵי לְהַרְחִיק אֶת הָאָדָם מִן הָעֲבֵרָה:
From when, that is, from what time, does one recite Shema in the evening? From the time when the priests enter to partake of their teruma. Until when does the time for the recitation of the evening Shema extend? Until the end of the first watch. That is the statement of Rabbi Eliezer.
The Rabbis say: The time for the recitation of the evening Shema is until midnight. Rabban Gamliel says: One may recite Shema until dawn. There was an incident where Rabban Gamliel’s sons returned very late from a wedding hall. They said to him, as they had been preoccupied with celebrating with the groom and bride: We did not recite Shema. He said to them: If the dawn has not yet arrived, you are obligated to recite Shema and that it is not only with regard to the halakha of the recitation of Shema, but rather, wherever the Sages say until midnight, the mitzva may be performed until dawn. If so, why did the Sages say that they may be eaten only until midnight? This is in order to distance a person from transgression, as if one believes that he has until dawn to perform the mitzva, he might be negligent and postpone it until the opportunity to perform the mitzva has passed.
(ב) מֵאֵימָתַי קוֹרִין אֶת שְׁמַע בְּשַׁחֲרִית. מִשֶּׁיַּכִּיר בֵּין תְּכֵלֶת לְלָבָן. רַבִּי אֱלִיעֶזֶר אוֹמֵר, בֵּין תְּכֵלֶת לְכַרְתִּי. וְגוֹמְרָהּ עַד הָנֵץ הַחַמָּה. רַבִּי יְהוֹשֻׁעַ אוֹמֵר, עַד שָׁלֹשׁ שָׁעוֹת, שֶׁכֵּן דֶּרֶךְ בְּנֵי מְלָכִים לַעֲמֹד בְּשָׁלֹשׁ שָׁעוֹת. הַקּוֹרֵא מִכָּאן וְאֵילָךְ לֹא הִפְסִיד, כְּאָדָם הַקּוֹרֵא בַתּוֹרָה:
(2) From when does one recite Shema in the morning? From when a person can distinguish between sky-blue [tekhelet] and white.
Rabbi Eliezer says: From when one can distinguish between sky-blue and leek-green.
And one must finish reciting Shema until the end of the period when you rise, i.e., sunrise, when the sun begins to shine.
Rabbi Yehoshua says: One may recite the morning Shema until three hours of the day, which this is still considered when you rise, as that is the habit of kings to rise from their sleep at three hours of the day. While there is a set time frame for the recitation of Shema, one who recites Shema from that time onward loses nothing. Although he does not fulfill the mitzva of reciting of Shema at its appointed time, he is nevertheless considered like one who reads the Torah, and is rewarded accordingly.