From when do we read Shema in the evenings? “From the time when people come [home] to eat their bread on Shabbat (Sabbath) nights,” [these are] the words of Rebbi Meir. And the Chachamim (Sages) say from the time when the Kohanim (priests) are able to eat their Terumah (heave offering). A sign for this is the coming out of the stars. And even though there is no explicit proof for this matter, there is an indirect reference: “… and half of them were holding spears from the morning twilight until the coming out of the stars.” (Nehemiah 4:15)
Rebbi Shimon says, “Sometimes a person may read it (i.e. The Shema) two times in one night, once before dawn and once after dawn. [And if that happens] it comes out that he fulfills his obligation for the day [reading] and for the night [reading].”...
Rebbi says, “There are four watches in a night. [There is a unit of time called] time period (Onah) which is 1/24th of an hour. And [there is another unit of time called] the time (Et) which is 1/24th of the time period (Onah). And [there is a third unit of time called] the moment (Regah) which is 1/24th of the time (Et).” Rebbi Natan says, “There are three watches in a night, as it is said ‘… the beginning of the middle watch.’ (Judges 7:19) There is no [such thing as a] middle, but only when there is something before it and after it.”