Dear Friend,

Mitzvot – Commandments, come in two types; “d’Oreisah” – Divine Decree and “d’Rabbonon” – Rabbinic decree. The recitation of the Shema is d’Oreisah because Torah instructs us to recite it “when thou liest down and when thou risest up”.

When we recite it…the time of day and evening is b’Rabbonon. That is to say, the rabbis of the Senhedrin concluded that “when thou liest down” is at a time, after darkness sets in, when the Kohain had been ritually purified and ate his offering.
“When thou risest up” was concluded as meaning to be the time of morning when there was just enough natural light to be able to see the blue strings among the white in the tallit. It was, later, concluded that, princes and kings trended to arise later than most people and, since we are all of kingly lineage, it was appropriate to delay the morning Shema until one could see the difference between the blue strings and something green (i.e. grass, leaves, etc.)

With the advent of time-measuring instruments, the Mitzvah b’Oreisah remains unchanged…Torah is complete and inalterable. However, the rulings of the rabbis can change…and did. It is now appropriate to recite the morning Shema by not later than 9:20 AM and the evening Shema up to the time that one goes to bed.

This is merely a brief response to your question and can be elaborated on in another issue. If you are seriously interested in reading the explanation of this or of any mitzvoth, look to the Mishna for your answers.