G-d told Aharon, “Bring your fellow Levites from your ancestral tribe to join you and assist you when you and your sons minister before the Tent of Meeting” (Bamidbar 18:2). What will you be doing there? You will be guarding the Temple. But why would G-d’s Temple require guards? This “guarding” was to show the proper respect due to the Temple. In fact, this guard duty was considered one of the sacred services performed by the Kohanim and Leviim, and the Kohanim wore their priestly garments when they carried it out. (They would change out of the garments when sleeping between shifts, as it was forbidden to sleep while wearing them.) Since guard duty was considered a priestly service, some say that children could not take part in it, and that the watchmen had to be twenty or older. Even at the age of twenty, Kohanim and Leviim were not authorized to perform all the services, but they were permitted to do this.
Because guard duty was a type of divine service, it should have been performed while standing. However, because it was for an extended period of time, the watchmen were permitted to sit when they were tired (though not to sleep, of course). In general, sitting in the courtyard of the Temple was not allowed, but in this case it was allowed as it was to enable the proper guarding of the Temple.
There is a disagreement as to the extent of the guarding. The Rambam says it was done at night only. However, according to some commentaries on Mishnah Tamid, the guarding was done around the clock. Others distinguish between the different places that were guarded saying that some areas were guarded around the clock, while others were guarded only part of the time. Within this opinion, there is a disagreement about whether the part-time guarding was during the day or night.