You have three hours after dawn to say the morning Kriat Shema.  This is because this is the latest time that princes used to wake up.

REMEMBER, the mitzvah is to read Kriat Shema "when you get up."  But "when you get up" is when most people normally get up.  Princes have no job and nothing to worry about.  So they sleep the lastest of anyone. 

But after that time can no longer be considered "when you get up."

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