I forgot who exactly it was but I believe it was during the time of the Vilna Gaon that a Rav went around Europe convincing people they should wear tzitzis all day because as music pointed out, its an easy mitzvah to do and you get schar for every moment.
Before that time, it was uncommon for people to wear tzitzis all day.
The mitzvah of wearing tzitzis starts when you wake up in the morning and ends at night.
There is an inyan to wear tzitzis at night/while sleeping for shmira.