Chasidut for Eruvin 129:21

[C] The main [purpose] of the recitation of the Shema is accepting the yoke of the Kingdom of Heaven and the yoke of Torah and the Commandments, as is taught (Berakhot 13a), and it must be done in the day and in the night - not in the morning and in the evening, but "When you lie down and when you arise," ibid., [Deut.?] 11:1 . It [the Torah] gives this time for this because the morning and the evening are changes in the world and not in the person, and it would have been enough to accept [the yokes] once for both of them, except for the changes in the person, who during the day arises and deals with his work, and must accept the yoke of the Kingdom of Heaven - that all his deeds be for the sake of Heaven, as written (Eruvin 65a), we are day laborers who are like laborers who labor that all their deeds are for their master, and even shorten Grace After Meals [to return to] his labor. And at night, the time for lying down and resting, there is also a need for accepting the yoke of the Kingdom of Heaven, because also in lying down on his bed he must know before Whom he lies down, as is written in the note [by Rabbi Moshe Isreles] at the beginning of Orach Hayyim [in the Shulhan Arukh]. And this is a greater labor. And as I heard on this [matter] according to "And Jacob dreamed," that from dreams a person's stature is made clear, although even if he does nothing, his ideas are only "And here the Lord was standing upon him." And on this matter there are few righteous people who merited such splendor. And for this reason, they gave the additional blessing for the evening prayer, which is Hashkivenu le-Shalom.