Mishnah Sources: berakhot 1:1, Kehati, Bavli, ADRN

What fence did the Sages make to their words? They ruled that the evening Shema‘ may be recited only until midnight, although Rabban Gamaliel said: Until cockcrow.⁹²Cf. the opening paragraph of Mishnah Berakoth. The Sages were also of the opinion that by strict law the Shema‘ may be recited at any time until the morning, but as a precautionary measure they ruled only until midnight. This is an example of a ‘fence’ by the Sages. How is this? When a man comes home from his work, let him not say, ‘I will eat a little, drink a little, sleep a little, and then I will recite the Shema‘’; it may result in his sleeping through the whole night and he will have missed reciting it. He should rather act thus: when he comes home from his work in the evening let him go to the Synagogue or the House of Study. If he is in the habit of reading Scripture, let him read it; if he is in the habit of studying the Mishnah, let him study it; otherwise, let him recite the Shema‘ and say the Tefillah; and whosoever transgresses the words of the Sages is deserving of death.⁹³Cf. Ber. 4b (Sonc. ed., p. 13).
Rabban Gamaliel said: There are times when a man recites the Shema‘ twice in one night, once before the rise of dawn and once after the rise of dawn, and he thereby fulfils his obligation both for the day and for the night.⁹⁴Cf. Ber. 8b (Sonc. ed., p. 44).
The Sages rose up and often made fences to their words.⁹⁵Cf. Finkelstein, op. cit., pp. 26-29 for an original and interesting interpretation of this sentence.

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