About a week ago, I saw in a siddur (might have been Koren) an instruction above the 1st Shema said as part of Birkot Hashachar stating that one should recite the 1st paragraph Ve'ahavta at this point if one feels that he will not be able to recite all 3 paragraphs before the time deadline for the morning Shema.
This had me thinking - does one fulfill the mitzvah of Shema by just reciting the first paragraph? Can he do this lechatchila? E.g. - he arises early in the morning, says the 1st paragraph and goes to sleep and when he awakens it is past the deadline?
Likewise for the night time Shema - he didn't say it earlier in the night, he awakens, says 1 paragraph, falls asleep and when he awakens, it's daytime.
If, in fact, the mitzvah can be fulfilled by just 1 paragraph then whey do we need to say the other 2?