Low watch

The low watch (or night watch) is part of the duty shift rotation on Federation starships and outposts, as well as other races and cultures' analogous organizations. 

History and specifics
The observance of a night cycle is necessary for many species that have diurnal rhythms, so many of the senior staff have sleep schedules that allow for a shift to take over the watch while they rest.

The night-shift commander is often referred to as the executive officer of the watch, although the position is not necessarily held by the first officer (who is also "executive officer" in title). (Star Trek: New Frontier)

At some Starfleet postings, night shift is the gamma shift, although some commanders order a four-watch rotation that also has time covered by delta shift. 

In the 2260s decade, aboard the USS Cochise (NCC-530), the low watch was manned by Lieutenant, Communications Officer Grayson and Helmsman Kronin.