Characters work six hour shifts unless their physiology dictates that they should work more or less, and the request to do so is approved by a superior officer. Generally, to fit into the 24-hour day that is followed most of the time (unless in the orbit of Bajor for example,) they will work one six hour shift in a day. There are specific shift patterns that the majority of officers follow, but flexible working requests are also available if a person prefers something different.
Alpha shift is usually ran by the likes of the senior staff as this is when they can be found on the bridge. Gamma and Delta shift are often ran by the ‘lower deckers’ as this is the quiet time, but it gives them the opportunity to work in higher positions that they might not usually get to do, such as working on the bridge.
Alpha: 0800 - 1400
Beta: 1400 - 2000
Gamma: 2000 - 0200
Delta: 0200-0800