The Mishnah Says:
The reference to “sunrise” does not imply that our mishnah follows the view of Rabbi Eliezer that the deadline for reciting the morning Shema is sunrise. (see mishnah 1.2) Rather, our mishnah refers to the practice of the vathiqin (the scrupulous ones), who always time their recitation of the Shema to finish with the rise of the sun. ([…])
The basis for this practice is the verse (Tehillim – Psalms 72.5): […] They will fear You with [the rising of] the sun, which is understood to mean that we should demonstrate our awe of G-D — by accepting His sovereignty with our recitation of the Shema — at sunrise.
The Mishnah Seder Zeraim Vol. I, Yad Avraham 3.5, Page 157 Artscroll Mishnah Series.