According to Kabbalah, one should wear a tallis katan, colloquially referred to as tzitzis, also during the night when a person sleeps. There are non-Kabbalistic reasons for this as well. For example, if a person were to go to sleep without tzitzis and sleep into the daytime, part of the day would pass without him wearing tzitzis. Also, if he wished to put on tzitzis immediately upon awakening, he would have to touch his tzitzis before he washed negel vasser. For these reasons and others, it is important to have available a pair of tzitzis to wear when one goes to sleep for the night. However, the Tzemach Tzedek points out that one should not designate the tzitzis specifically for the night, because then one would be wearing a k’sus leilah, a night garment, and according to many opinions night garments do not require tzitzis. Accordingly, the tzitzis on this garment would not be considered to have the sanctity of tzitzis and he would not be considered fulfilling the mitzvah. The pair of tzitzis should therefore be for use during both day and night, and since it is used for both day and night, it requires tzitzis. The fact that the wearer sleeps in the tzitzis into the morning does not make it a day garment; it should be a pair of tzitzis that is worn both day and night. The common practice of having a night tzitzis only is not correct. One should therefore change their tzitzis in the morning in order to recite a brachah on a different pair and wear the same pair of tzitzis until the following morning. The brachah on the tzitzis is said only in the morning, even if one puts on a different pair of tzitzis at night.