Tosfos (8b DH Lo) ask why Rebbi Shimon bar Yochai permits the night Shema before Amud ha'Shachar and the day one afterwards? Having concluded that after Amud ha'Shachar is considered nighttime, why did he not permit the recital of both Shema's after Amud ha'Shachar (which is a bigger Chidush)?

And Tosfos answers that the conclusion that after Amud ha'Shachar is still considered nighttime is a Chumra, which precludes the performance of Mitzvos that can only be performed by day, such as Tefilin and Tzitzis. It is not nighttime as far as the Shema is concerned, since some people are already getting up (so it cannot be considered Zman Shechivah [lying-down time]).

The next Tosfos follows the same pattern. It asks why after having concluded that before Hanetz ha'Chamah is considered daytime, Rebbi Shimon did not permit the recital of both Shema's during that period? The answer is that it is only considered daytime with regard to the Mitzvos (Tefilin and Tzitzis), but not with regard to the Shema, seeing as some people are still lying down (so it cannot be considered Zeman Kimah (getting-up time).

Be'Virchas Kol Tuv

Eliezer Chrysler