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,
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.