From what time do we read the shema at night? From the time that the Kohanim (priests) enter to eat their Trumah (holy food). Kohanim that became defiled, and then immersed (in the mikvah), and the day has become night [resetting their purity], then they are allowed to eat Trumah.

UNTIL THE END OF THE FIRST WATCH- A third of the night, as will be explained in the Gemara (p. 3a), and from then on the time has passed for it is not the time of lying down, and is not referred to in the words "Bishachbicha". Also, before this period is not considered the time for lying down. Therefore, if one read the Shema before, he does not fulfill his obligation. If so, why do we read the Shema in the Synagogue? In order to stand in Tefilah, in connection with words of Torah as is taught in the Briata in the Jerusalem Talmud Berachot. Therefore one would have to read again after it gets dark. [In this case] Reading the first section of the Shema at bed time suffices.

Until dawn - The entire night is called the time of laying down.

The Burning of the fats and limbs - Of the sacrifices that their blood is sprinkled during the day.

Their commandment - is to be brought the entire night and they do not become invalidated through "sleeping" until the morning star rises

Fats - of all (types) of sacrifices

Limbs - of a burnt offering

And all (offerings) which must be eaten that same day - for example sin offerings and guilt offerings and the lambs of atzeres and meal offerings and thanksgiving offerings

Their commandment - ie the allotted time for their being eaten

Until Dawn - And this is what brings them to become Nossar (leftover) as it says with the Todah Sacrifice "A person should not leave anything over until morning" and all of this is learnt from the Todah sacrifice.

If so why did the sages say until midnight - This is regarding Kerias Shema and eating Kodshim.

In order to distance a man from sin - And they are forbidden to eat before their time, in order that they should not come to eat them after dawn and therefore he would be liable to Kores. Similarly with reading the Shema, this is to encourage a person, so that he should not say "I still have time" and in the meantime it will get dawn and he would have missed the time. The offering of the fats that we learn here the sages did not say until midnight at all, and we did not learn here only to teach us that anything that is applied to at night is valid the entire night. Here also we learnt in a mishnah in the second chapter of Megillah. "The whole night is valid for reaping the omer and to offer up the fats and the limbs".

How is this possible - From where does the Tanna conclude the obligation to read Kerias Shema, that he began to ask here the time of reading?

On the text it is based - And there you can learn the obligation of reading (Shema)

If you want I can say - the reason we learn evening first is to learn it from the creation of the world.

It then taught about the morning - "From when can we read the shema in the morning".

From nightfall - This is when the sun has completely set as we learn later on.