On board a ship, it is a requirement that someone is awake at all times. Especially at sea, where the ship is in motion, there are usually multiple people awake to ensure the smooth operation of the vessel.

There are several methods of scheduling the watch. The simplest one, most hated by sailors is the 7-5-5-7 system. In this schedule, each watch is on duty for seven hours, off duty for five, on duty for five, and off duty for seven. This breaks down to twelve hours a day on and twelve off, but you will always have the same watch. If you start on nights, you will remain on nights for the duration of the rotation, often until the ship reaches harbour.

The preferred watch system consists of seven 4-hour watches and two 2-hour watches (in military time):

The purpose of the dogwatches are to ensure that the night watch rotates around the different watches.

Traditionally, the watches are signalled by the ship's bell.