מילון על ברכות 1:2

UNTIL THE END OF THE FIRST WATCH- A third of the night, as will be explained in the Gemara (p. 3a), and from then on the time has passed for it is not the time of lying down, and is not referred to in the words "Bishachbicha". Also, before this period is not considered the time for lying down. Therefore, if one read the Shema before, he does not fulfill his obligation. If so, why do we read the Shema in the Synagogue? In order to stand in Tefilah, in connection with words of Torah as is taught in the Briata in the Jerusalem Talmud Berachot. Therefore one would have to read again after it gets dark. [In this case] Reading the first section of the Shema at bed time suffices.