Q. If someone wears the same garment day and night, is that okay, or it becomes a night garment that is exempt?

A. According to the opinion of the Rambam (beginning of Hilchos Tzitzis) it matters not what type of garment you wear, as would be a day garment or an article of clothing dedicated for night use, but rather when you wear it. If it is day you comply with the mitzvah of Tzitzis if night you do not. However Rosh’s opinion is that it does depend on the type of garment, and a vestment designated for day use would be liable to tzitzis even at night.. Both opinions are mentioned in Shulchan Aruch (O.H. 18: 1) and we follow the stringency of both (Remoh ibid.)

Horav Shlomo Miller’s Shlit”a opinion is that a garment worn constantly day and night, is considered a day garment and is liable to tzitzis at night according to the Rosh’s opinion. Therefore it would make more sense, for someone who has a tradition of sleeping with a taalis kattan, to wear a tzitzis garment day and night (until it requires laundering), rather than having a dedicated night garment. As in the first case at least you comply with the mitzvah following the Rosh’s opinion.

However, since the tzitzis garment was not removed at night, you would not recite a brocho on it in the morning, but rather have it in mind when reciting the brocho for the talis. (Mishna Berura 8: 42)

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a