Rambam: Tzitzit, Chapter 3

It's not only permitted to wear tzitzit at night but even recommended, according to the mystical approach of the Arizal. This is for various reasons, including for spiritual protection from negative forces.

See here for an in-depth article on the subject..


Note:
The potential issue with wearing tzitzit at night is only for a garment that contains a mixture of wool and linen ('shatnez'), which wouldn't be permitted to wear unless one were fully obligated to do so. However, nowadays this is not an issue because we anyways don't wear any tzitzit containing this mixture. Reply