Rabbi Yehoshua says in the Mishna that kerias shema may be recited until three hours into the day, since the princes are still sleeping. The Rashba above (8b) asks  from the Gemora there that says that one can fulfill the mitzvah of kerias shema of the evening, even if it is recited after dawn, but before sunrise. The rationale is that although most people are awake by then, and it is already considered day, there are still people sleeping then, and therefore, it is still in the time of "u'v'shachb'cha." If so, the Rashba asks, why can't kerias shema be recited even after sunrise, until three hours into the day? The princes are still sleeping at that time!

He answers that the princes are a "minority of a minority," and therefore, it cannot be regarded as a time that people are still sleeping.

If so, he asks, why is the time for kerias shema in the morning extended because of them?

He answers that since it is daytime, it is logical to extend the time for the morning kerias shema, as long as people are still sleeping (even though, the princes are a "minority of a minority"). However, in regards to the evening kerias shema, since after sunrise it is considered daytime, we cannot use the "princes sleeping" to extend the time for the fulfillment of the evening kerias shema. If many people are sleeping, it is still regarded as the time of "u'v'shachb'cha," but not if it's only a "minority of a minority."

The Ramban answers the original question by saying that the princes are actually awake from before; however, they remain in their beds until the third hour of the day.

The Magen Avraham asks on the Ramban from the Gemora above (3b), where Dovid said that his eyes precede those of the other kings by two mishmoros (eight hours). According to the Ramban, it would only be six hours!

Mishmeres Moed answers that Dovid's eyes preceded the time that the other kings arose from their bed. He didn't say that he was awake eight hours before them; he was saying that he arose to study Torah eight hours before they got up to engage in their activities.