“ Rabbi Yehoshua’s view is that “when you arise” means “when everyone is awake,” and since there are some people (namely, princes) who sleep in until one-quarter of the day has passed, the time until they wake up can still be considered “when you arise.” [The halachah follows Rabbi Yehoshua’s opinion, although it is preferable to act according to Rabbi Eliezer and to recite the Shema starting a few minutes before sunrise — Maimonides, in his Code (Laws of Reciting the Shema 1:11) , gives a figure of about six minutes — so that one completes it exactly at sunrise.] ”

“ Once there is enough (natural) illumination to distinguish between the tzitzit threads, then, the (anonymous) authority of our mishnah considers it to be the time when people are starting to wake up (“when you arise”) . ”