Nachschlagewerk zu Berakhot 1:12

Berachoth: Chapter One From what time on may we read the Sh'm'a of the evening? "From the time the priests enter to eat their Terumah until the end of the first watch," said R. Eliezer. But the other sages say "Until midnight," and Rabban Gamaliel says "Until the appearance of the morning star." It happened that the sons of Rabban Gamaliel came [very late at night] from a banquet and told Rabban Gamaliel that they had not yet read the Sh'm'a, whereupon he said to them: "If the morning star has not yet appeared you must read it."