It is a positive biblical commandment to recite Shema at night and in the morning, as it is written (Deuteronomy 6:7), “And you shall say them… when you lie down and when you get up.” “When you lie down” denotes nighttime, and “when you get up” refers to morning.
We read three paragraphs, the first: “Shema Yisrael” (“Hear O Israel”) (Deuteronomy 6:4-9), with which we accept the yoke of Heaven, and which discusses the unity and love of Hashem. The second is “V’hayah Im Shamo’a” (“And if you follow”) (Deuteronomy 11:13-21), which includes the acceptance of the yoke of the mitzvot. The third is “Vayomer” (“And He said”) (Numbers 15:37-41), including a command to remember the mitzvot through the commandment of tzitzit, and mention of the Exodus from Egypt.
The Chachamim arranged the Shema paragraph to precede V’hayah Im Shamo’a so that a person will first accept the yoke of Heaven, and only afterwards accept the yoke of the mitzvot. They also placed V’hayah Im Shamo’a which involves a general mandate to keep all the mitzvot, including those performed during both day and night, before the Vayomer paragraph, which discusses the mitzvah of tzitzit, performed only during the day (Berachot 13a).
In the opinion of some Rishonim, the biblical commandment is just to recite the verse “Shema Yisrael.” According to them, this is the meaning of (Deuteronomy 6:6-7), “These words I am commanding you today must be in your heart…when you lie down and when you get up.” However, the Chachamim instituted reciting the entire three paragraphs.
It can be explained, that the essence of the mitzvah of Keriat Shema is indeed that a person accepts upon himself the yoke of Heaven, and for that reason, even one who only recites the first verse fulfills the biblical commandment. Yet, the more a person enhances his acceptance of the yoke of Heaven, the more completely he fulfills the biblical commandment. Therefore, the Chachamim instituted the recital of all three paragraphs, for within them are the fundamentals of faith, the acceptance of the yoke of the mitzvot, and the remembrance of all the mitzvot through the mitzvah of tzitzit. Hence, in actuality, we fulfill the biblical mitzvah by reciting all three paragraphs.