Related על ברכות 1:2

Rebbi Shimon says, “Sometimes a person may read it (i.e. The Shema) two times in one night, once before dawn and once after dawn. [And if that happens] it comes out that he fulfills his obligation for the day [reading] and for the night [reading].”...

From when do we read the Shema in the morning? Acherim (Others) say, “[from the time] when his friend will be four amot away from him and he recognizes him.” The mitzvah [to say it] is with sunrise in order to connect redemption (Geulah) to prayer (Tefillah), and it will come out that he will pray during the day. Rebbi Yehudah said, “One time I was walking behind Rebbi Akiva and Rebbi Elazar Ben Azaryah and the time came to read the Shema. I supposed they gave up on reading the Shema [right away] because they were busy with the needs of the community. I read [the Shema] and [then] learned [some Torah] and after that they began [reading the Shema], and the sun could already be seen above the mountain tops.”