The Talmud exempts one from the obligation of tzitzit at night, as a consequence of the requirement of “And you shall see them”. As noted by the Shulchan Arukh (O.C. 18:1), there are two ways in rishonim to understand this exclusion: that of the Rosh, who exempts nighttime clothing, but not daytime clothing worn at night; and that of the Rambam, who refers not to the type of clothing but to the actual time of day.

R. Tzvi Pesach Frank (Resp. Har Tzvi, O.C. 12) considers these views in analyzing a statement of Rashi (Sefer HaPardes, Hil. Tzitzit 36), that in the presence of candlelight, one “may” wear tzitzit at night. He notes that statement of the Hagahot Maymoniot (Hil. Tzitzit ch. 3) that Rashi agrees with the Rosh; he observes the difficulty involved, as light would add nothing if daytime clothes are obligated in any event. He suggests that rather, Rashi agrees with the Rambam, but considers the possibility that candlelight may create circumstances of daytime. Thus, Rashi “allows” one to take into account this safek, and not be concerned with the prohibition of bal tosif.

R. Frank concludes his responsum by disputing some suggestions of his questioner, arguing that a darkened room during the day would not exempt one from tziztit, and that the possibility of being obligated at night (either because of candlelight or daytime clothing) would not affect the status of “mitzvat aseh she-hazman grama” (in that the obligation still stems from the aspect of daytime; see Tosafot, Kiddushin 35, s.. u’tefilin).