Berakhot 4B

ואי כרבן גמליאל סבירא להו ⁠— לימרו כרבן גמליאל.

And if they explain this verse in accordance with the opinion of Rabban Gamliel who says that “when you lie down” refers to the entire night, then let the Rabbis also say that one may recite the evening Shema until dawn, in accordance with the opinion of Rabban Gamliel.

לעולם כרבן גמליאל סבירא להו, והא דקא אמרי ״עד חצות״, כדי להרחיק את האדם מן העבירה. כדתניא, חכמים עשו סייג לדבריהם, כדי שלא יהא אדם בא מן השדה בערב, ואומר: ״אלך לביתי ואוכל קימעא ואשתה קימעא, ואישן קימעא, ואחר כך אקרא קריאת שמע ואתפלל״, וחוטפתו שינה ונמצא ישן כל הלילה. אבל אדם בא מן השדה בערב, נכנס לבית הכנסת, אם רגיל לקרות ⁠— קורא. ואם רגיל לשנות ⁠— שונה, וקורא קריאת שמע ומתפלל, ואוכל פתו ומברך.

The Talmud answers: Actually, the Rabbis hold in accordance with the opinion of Rabban Gamliel, and the fact that they say until midnight is in order to distance a person from transgression. As it was taught in a baraita, the Rabbis created a “fence” for their pronouncements with regard to the recitation of Shema in order to prevent a situation where a person comes home from the field in the evening, tired from his day’s work, and knowing that he is permitted to recite Shema until dawn says to himself: I will go home, eat a little, drink a little, sleep a little and then I will recite Shema and recite the evening prayer. In the meantime, he is overcome by sleep and ends up sleeping all night. However, since one is concerned lest he fall asleep and fail to wake up before midnight in order to recite Shema at the appropriate time, he will come from the field in the evening, enter the synagogue, and until it is time to pray, he will immerse himself in Torah. If he is accustomed to reading the Bible, he reads. If he is accustomed to learning mishnayot, a more advanced level of study, he learns. And then he recites Shema and prays as he should. When he arrives home, he eats his meal with a contented heart and recites a blessing.