Berachos, Chapter 1, Mishnah 2. Our Mishnah raises with regard to the morning Shema the very same question that was asked in the first Mishnah with regard to the evening Shema: from what point in the morning may one first recite Shema?
The beginning of the time for the morning Shema:
"Mishe'yakir bein techeiles le'lavan" - this is the opinion of the first Tanna in the Mishnah: from the moment a person can recognize and distinguish between the techeiles shade in the tzitzis (the wool dyed a bluish shade) and the white in it.
The opinion of the Baraisa - "there are those who say": from when one can see his fellow at a distance of four amos and recognize him. The Yerushalmi clarifies that these two recognitions - distinguishing between techeiles and white, and recognizing a fellow's face - occur at precisely the same time.
"Bein techeiles le'karti" - Rabbi Eliezer sets the time slightly later: one must be able to distinguish between techeiles and karti, which is a blue shade tending toward green. Distinguishing between these two similar shades is more difficult and requires more light, and therefore its time is later.
The end of the time for Shema:
The Mishnah adds that one must complete the recitation by sunrise - this is the opinion of the first Tanna, the first view. We do not rule this way in practice, and although it is proper to do so ideally, there is no obligation from the basic law.
Rabbi Yehoshua says: "ad shalosh sha'os" - until three hours into the day, and this is our practice. Understandably, this refers to proportional hours: the day is divided into twelve equal parts, and therefore in the winter the proportional hours are shorter, and in the summer they are longer.
The reason for setting three hours: princes, the sons of kings, who live lives of leisure and are not required to rise early, get out of bed at the latest by the end of three hours into the day. It emerges that until this hour the matter is still considered within the category of "u'v'kumecha" - since there are still people who rise at this hour.
"Lo hifsid":
The Mishnah establishes that one who recites from this point onward "lo hifsid, ke'adam ha'korei ba'Torah" - he has lost nothing, like a person reading from the Torah. The Gemara explains that the intent is not merely that he loses nothing by reciting it, but that he may even recite the berachos of Shema that surround it. However, once he has reached a later time than that - one additional proportional hour - there is already a serious question in halachah whether he may recite the berachos of Shema at all.
The question of the Magen Avraham:
The Magen Avraham raises a fascinating question: if we rule like Rabban Gamliel that "u'v'shochbecha" means the entire time that people sleep, why should "u'v'kumecha" not likewise be interpreted as the entire time that a person is awake, according to that very same opinion? Yet no one holds this way.
The Magen Avraham explains that "u'v'kumecha" differs in its definition from "u'v'shochbecha": one who lies down remains lying down throughout the entire time of his sleep, whereas a person who is awake adopts various positions throughout the day - he sits, stands and walks. Therefore "u'v'kumecha" means at the time of rising itself, and not the entire time that he is awake.
It is worth noting that the Magen Avraham cites the opinion of the Minchas Kohen, who because of this difficulty held that "u'v'kumecha" indeed means the entire time that a person is awake, and therefore he was of the view that the prohibition of reciting Shema after three hours into the day is only a rabbinic law. However, we do not rule this way, but rather like the opinion of the Magen Avraham, that the matter is from the Torah.
In summary: In this Mishnah we learned the times for the morning Shema - it begins from when one can distinguish between techeiles and white (and according to the Baraisa, from when one can recognize a fellow at a distance of four amos, and as the Yerushalmi says these are one and the same time), and according to Rabbi Eliezer from when one can distinguish between techeiles and karti. It ends, according to the first Tanna, at sunrise, and the halachah follows Rabbi Yehoshua, until three proportional hours, the time when princes rise. We also learned that one who recites afterward "has lost nothing" and may even recite its berachos, and we examined the question of the Magen Avraham regarding the definition of "u'v'kumecha" as opposed to "u'v'shochbecha", and the dispute whether the limit of three hours is from the Torah or rabbinic.