from Rav Pam's Haschalas Hashas (last Machzor)

Rav Pam TZ"L from the 11th Machzor Hascholos Hashas: The Avnei Nezer asks, why doesn't the Mishna say simply that you read Kris Shma from Tzaes Hacochavim? He answers to compare K"S to Tumah, just like someone who is Tameh during the day, even after going to the Mikvah , doesn't become Tahor until nightfall, because the Roshum of the tumah lasts the whole day, so too K"S isn't repeated until nightfall beacuse the effects of the morning K"S has not worn off until then. Rav Pam added that the same is true for daf yomi, that the roshum of the daf lasts the whole day ( I was told that he added that therefore you should learn the daf in the morning).