[Even during the daytime, the Torah command to wear tzitzit only pertains to one who is already wearing a four-cornered garment. There is no obligation to actually wear a four-cornered garment. (This can be compared to the mitzvah of shechitah (slaughtering). There is a commandment to slaughter an animal if you want to eat meat. However, there is no obligation to eat meat). At night, however, according to biblical law it would be permitted to wear an "un-tzitzited" four-cornered garment. The Rabbis however, forbade wearing such a garment even during nighttime, lest the person forget to remove the garment before dawn.] 

Chassidim are careful to wear tzitzit even during the night, and even while sleeping. There are several reasons for this practice: (a) according to certain opinions, one can fulfill the mitzvah of tzitzit during the night if wearing a designated day garment. (b) If waking up after daylight, one will have fulfilled the mitzvah of tzitzit during the morning hours which have elapsed before his wakening. (c) According to Kabbalah, tzitzit provide a measure of protection even after dark.

* The definition of “daytime” and “nighttime clothing” is the subject of dispute between the medieval Halachic scholars. One opinion is that any garment worn during daytime hours is obligated in tzitzit and any garment worn at night is exempt. Others maintain that only a garment designated for nighttime use, such as pajamas, is exempt from tzitzit—even during daylight hours. Conversely, a garment which is normally worn during daytime is obligated in tzitzit even during the nighttime.

The Halachah has not been established according to either opinion.