Related על ברכות 1:1

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.”

All of the day of the waving (i.e., the first day of the Omer, see Sukk. 3:12), it is forbidden to eat new grain. All of the seventh day [of Sukkot] one is obligated in sukkah, and all of the seventh day [of Sukkot] is valid for [waving] the lulav. All of the eighth day [after birth] is valid for circumcision. All of the night is valid for the reaping of the omer and for the burning of limbs and fat [leftover from the day's sacrifices]. The general principle is that anything commanded during the day is valid all day and anything commanded at night is valid all night. One is not liable for notar [leaving part of the shelamim sacrifice over] and the intention [at the time of the sacrifice to eat the shelamim sacrifice later] does not invalidate as pigul until dawn.