“ Yet in order to reduce the need for artificial light, some people during the Talmudic era would retire much earlier, at nightfall. Hence nightfall, when priests "go in to eat their terumah," is the earliest time for reciting the evening Shema.
By contrast, the Sages and Rabban Gamaliel agree that the verse means "at the time when people are asleep," which would mean the entire night. ”

“ This refers to the statement of the Sages earlier in this mishnah.
To distance a person from transgression: Evidently the Sages were concerned that if someone did not say the Shema before midnight, he may forget and lose track of time and accidently fail to perform this mitzvah. ”

“ These steps must be performed during the daytime; the fats and organs can be burned anytime during the night, but if any of them are left over by morning, they become invalidated and must be burned outside the Temple precincts. ”