Commentary for Berakhot 4:17

SAYS R’ YEHUDAH TO [R’ MEIR, HOW CAN YOU SAY THAT THE TIME FOR RECITING SHEMA IS WHEN THE KOHANIM IMMERSE THEMSELVES], BUT THE KOHANIM IMMERSE THEMSELVES WHEN IT IS STILL DAYTIME? R’ Meir’s challenge to R’ Yehudah indicates that it is inconceivable that the time for reciting the evening Shema is when it is still daytime. Tosfos’ finds this statement by R’ Yehudah extremely perplexing. This is bewildering! [R’ Yehudah] should ask the very same question to himself? For the half minchah¹See Artscroll 26b notes 28 and 29, for a definition of half minchah. According to R’ Yehudah when the half minchah passes one can no longer say the minchah prayers and may recite maariv prayers. At that time he will recite the evening Shema. On a day when the stars appear at six p.m. the half minchah will be at four forty five p.m. when the sun is still shining. is still during the day and [R’ Yehudah] says (below 27a) that immediately after the half minchah passes is the time for evening prayers and we recite Shema while the sun is still brightly shining. According to Tosfos, if the stars appear at six p.m. the half minchah is approximately at four forty five p.m. The last time for immersion would be at approximately five forty five p.m. R’ Yehudah’s recital time is almost an hour earlier than R’ Meir’s. Why does he have a difficulty with R’ Meir’s opinion that the time for reciting Shema is before sunset B)? His own opinion is that the time for reciting Shema is even earlier.
There must be a fundamental difference between R’ Meir and R’ Yehudah about the time for reciting Shema. We can answer: That according to [R’ Yehudah] there is no difficulty with reciting while it is still daytime, because he does not expound the words ובשכבך ובקומך-when you lie down to sleep and when you arise²How does R’ Yehudah explain the words ובשכבך ובקומך? Some suggest that R’ Yehudah follows his teacher R’ Tarfon who holds like Bais Shamai that ובשכבך ובקומך refers to the position that one must be in when reciting the Shema. During the evening Shema one must recline and during the morning Shema one must stand. as defining the time to recite Shema. The Mitzvah is dependent on day or night and the half minchah is the beginning of night insofar as the order of prayers is concerned. However, it is definitely not the time for lying down to sleep.
However, according to those Rabanan who do expound the verse ובשכבך ובקומך as the source for the time to recite Shema, there is a difficulty, because the time of immersion is definitely not the time when people lie down to sleep.