May one have a separate pair of sleeping Tzitzis to wear only at night?
Although it is proper for those who sleep with a Tallis Katan to have two pairs of a Tallis Katan in order to be able to switch the Tzitzis daily upon awakening and hence recite a new blessing, nevertheless one should not designate one of these pairs to be worn only at night. This applies even though one wakes up in the morning wearing the night pair, if he is not accustomed to continue to wear it throughout the day. The reason for this is because one who has a night Tzitzis which he wears only at night is not fulfilling the Mitzvah of Tzitzis according to any opinion. Also, one who has a designated pair of night Tzitzis, according to some opinions would not be fulfilling the Mitzvah of Tzitzis even during the day, if he needed one day to wear it during the day. He would not be allowed to say a blessing on such Tzitzis.
The options: There are several options available to avoid the problem of having only night Tzitzis: 1) Never switch Tzitzis, and continue wearing the same pair by both night and day [although in such a case, he cannot say the blessing in the morning]; 2) Switch a pair every morning and sleep with the same pair at night. 3) Switch a pair every night and continue wearing it into the day. [However, it is better to follow the option of changing the pairs in the morning as opposed to at night in order to avoid entering oneself into a Safek Bracha.] 4) Have a designated night Tzitzis but make sure to designate it to also wear during the day on occasion. When one occasionally wears these Tzitzits also during the day, as he designated, then it is considered a night and day garment, which is obligated in Tzitzis. It is unclear however how often one must wear the Tzitzis during the day to consider it a day garment.
Background of Obligation of Tzitzis during nighttime:
The Mitzvah of Tzitzis does not apply at night, to night garments. The definition of “night garments” is disputed amongst the Poskim. Some Poskim rule that the above exemption refers to the actual period of nighttime, and hence the law means to say that the Mitzvah of Tzitzis does not apply to any garment that one wears at night, even if it is a garment that is normally worn during the day. Other Poskim however rule that the above law refers to a garment that is designated to be worn only at night, and hence the law means to say that such a garment is not obligated in Tzitzis even if one wears it on occasion during the day. On the other hand, according to this opinion, when wearing a garment that is designated to be worn only during the day, or is designated to be worn by both night and day, then one is obligated to attach Tzitzis to it and say a blessing upon wearing it even if one wears it at night. Practically, due to lack of arbitration in the above mentioned dispute, one is to be stringent like both opinions. Thus, one is to have Tzitzis on all garments that are designated to be worn during the day, or during the night and day, even if he is wearing them at night. Likewise, one is to tie Tzitzis onto all garments that are worn during the day, even if they are garments that are worn only during the night. [However, a garment that is designated to be worn only at night, is exempt from Tzitzis according to all when worn at night. Hence, night-only garments may be worn at night without Tzitzis.] The above stringency is only regarding the obligation to place Tzitzis on such garments, however regarding the blessing, one is not to say a blessing over Tzitzis unless he is wearing it during the day and it is also a garment that is designated to be worn during the day, or during the night and day.
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 Piskeiy Dinim Tzemach Tzedek 3/2; Ketzos Hashulchan 7 footnote 23; Ashel Avraham Butchach Tinyana 21; Rav Akiva Eiger 121/2; Toras Chaim Sofer 21/5; Darkei Chaim Veshalom 38; Implication of custom of Arizal brought in 21/15
The reason: As according to all opinions, a night Beged that is worn at night is exempt from Tzitzis and hence one is not fulfilling the Mitzvah according to any opinion. [Poskim ibid]
Other opinions and customs: Many are accustomed to have a Tallis Katan which they only wear upon sleeping at night. Some Poskim justify this custom, stating that the Segulos involved in wearing a Tallis Katan apply even if it is not Halachically obligated to have Tzitzis, being it is a night garment. [See Birchas Habayis 37/4; Baruch Taam on 21; Shevet Halevi 6/50; 10/2; Kinyan Torah 4/1; Piskeiy Teshuvos 21/5 footnote 74]
 Implication of Admur 18/5 and M”A 18/3 regarding blankets which are worn into the morning, and at times by day, that it is a dispute if they are required to have Tzitzis, hence implying that according to all they are defined as a Kesus Layla! Hence, when one wears it by day he cannot say a blessing, as according to the opinion that a Kesus Layla is exempt from Tzitzis, this garment is not obligated to have Tzitzis. Furthermore, if one did not remove this Tallis in the morning and then re-wear it, perhaps even according to the Rambam it is exempt from Tzitzis, and hence one is not wearing Tzitzis according to any opinion. [See Elya Raba ibid; implication from Siddur ibid, brought in previous footnotes]
 8/27; 494/3; Tur 8; Bach; Taz 8/15; M”A 8/21; Elya Raba 8/18; M”B 8/42; Custom of Arizal [as brought in Kaf Hachaim 8/61]; Chesed Lealafim 8/7; Ben Ish Chaiy Bereishis 2; Kaf Hachaim 8/61
Other opinions: Some Poskim rule that one who slept with a Tallis is to recite a blessing over it upon awaking in the morning. [Michaber 8/16] The Poskim ibid, negate this opinion, and so rules Admur, and so is the Sefardi ruling, unlike the Michaber, as Safek Brachos Lihakel. [See Kaf Hachaim ibid]
 The reason: As if one changes the pair at night he enters himself into a Safek Bracha, as some opinions rule one is to recite a blessing over day Tzitzis that are worn at night. Thus, it is better to change the Tzitzis by morning rather than at night even if he plans to wear the Tzitzis until the next night, and then switch it again. [See Taz 581/2; Mateh Ephraim 581/14; M”B 581/6; Piskeiy Teshuvos 18 footnote 16 in name of Maharshal 68 that it is forbidden to do a Mitzvah without a blessing See Tehila Ledavid 18; Piskeiy Hasiddur 42] In a reply from Harav Y.L. Groner I was told he did not receive any set directive in this matter; as to when one is to switch the Tzitzis, by night or by day. A similar reply was received from Harav Y.S. Ginzberg.
 Admur 18/1; brought also in 8/27; Michaber 18/1; Menachos 43a
The reason: As the verse states “Ureisem Oso” “That you shall see it” thus coming to exclude night garments. [Admur ibid; Menachos ibid]
At night near light: Some Poskim write that a four-cornered garment is obligated in Tzitzis at night if it is in an area near a candle, and hence can be seen. [Pardes of Rashi Tzitzis 36; Hagahos Chasam Sofer 18; See Har Tzevi 1/12]
 Rambam 3/8; first opinion in Admur and Michaber ibid
 The reason: As in their opinion the verse is excluding garments that cannot be seen while it is worn. [Admur ibid]
 Rosh Hilchos Ketanos Tzitzis 1 [p. 40]; 2nd opinion in Admur and Michaber ibid
 The reason: As initially the garment was made to be only worn at a time that it cannot be seen. [Admur ibid]
 Admur ibid; Levush; Olas Tamid 18/1 and 3; Tzemach Tzedek Piskei Dinim 18; Ben Ish Chaiy Lech Lecha 16; Kaf Hachaim 18/8
Other opinions: The Bach rules that if it is designated for day and night use, it has the same status as a night garment. The Poskim ibid negate this opinion.
 The reason: As these garments were made to be worn at a time that they are visible. [Admur ibid]
 Admur ibid and 19/2; Darkei Moshe; Olas Tamid 18/2; M”A 18/3 as explained in Machatzis Hashekel; Elya Raba 18/1; Ben Ish Chaiy Lech Lecha 16; Kaf Hachaim 18/9
Other opinions: In the Beis Yosef the Michaber rules like the Rambam. Nonetheless, the Sefardi custom follows the ruling in Admur, as Safek Brachos Lihakel is said even against Maran. [Birkeiy Yosef 7/3; Kaf Hachaim 18/7]
 Why does the wearing of such Tzitzis not constitute Baal Tosif according to those that exempt it from Tzitzis? As whenever one fulfills a Mitzvah due to doubt it is not considered Baal Tosif. See Siddur Admur “Doing so does not involve the prohibition of Baal Tosif being that there are opinions that state one is obligated to wear Tzitzis on a day garment even if it is worn at night.” See also Admur 31/2 regaridng Chol Hamoed and 34/4 regarding Rabbeinu Tam Tefillin and why it does not transgress Baal Tosif
 Regarding if wearing such a garment at night together with Tzitzis constitutes Baal Tosif-see Q&A!
 Admur ibid and 19/2; Rama ibid
The reason: As Safek Brachos Lihakel, and hence one must suspect for both opinions. [ibid]