Menachos 40b - Tzitzis at Night

Rav Zeira says that the reason for the Rabbonon uprooting the mitzvah of tzitzis on a linen garment is because it would lead to a violation of shatnez on a כסות לילה. Rashi explains that the concern is that one will wear the linen garment with his wool ticheiles even at night when there is no mitzvah of tzitzis thereby violating the prohibition of wearing kelaim even in the absence of a mitzvah of tzitzis. Rashi seems to hold like the opinion of the Rambam that everything depends on actual day and night. All garments are obligated in tzitzis by day and are exempt at night. However, Tosafos (d.h. mi'shum) says in the name of Rabbeinu Tam that the issue isn't dependent on actual day and night, rather a day garment is obligated in tzitzis even at night, whereas a night garment is exempt from tzitzis even during the day. Tosafos explains that even according to Rabbeinu Tam we regard the mitzvah of tzitzis as a time bound mitzvah from which women are exempt because ultimately the time is responsible for determining whether this particular garment is obligated in tzitzis. According to Rabbeinu Tam who considers everything to be dependent on the garment, there is no concern that one will wear a day garment of linen and ticheiles even at night, because according to R"T a day garment at night is obligated in tzitzis. The concern has to be that if we allow one to put ticheiles on a linen day garment, they will also come to put it on a linen night time garment.

Tosafos (d.h. ticheiles) writes that a linen garment with ticheiles can be worn even at a time and even by a person that is not fulfilling a mitzvah (such as a woman). Tosafos here seems to understand that a daytime garment is exempt from tzitzis at night, like rashi and not rabbeinu tam, yet when worn at night there is no violation of kelayim (against rashi).

The Sha'agas Aryeh (siman 30) asks on Rabbeinu Tam from the gemara in brachos which says that in ma'arava they didn't say the entire parsha of tzitzis at night (only the beginning and end) because there is no mitzvah of tzitzis at night. The Sha'agas Aryeh asks that the mitzvah of tzitzis applies by night as it does by day so long as it is a day garment, so we should be saying the parsha of tzitzis at night? One of the commentaries (printed in the back of the gemara) answers that since night garments are exempt, there is a precedent for the parsha of tzitzis being skipped at night - similar to what rabbeinu tam himself says to justify how it is a time bound mitzvah.