Where do the news anchors that report on "breaking news" come from at all hours of the day?

Whenever there is breaking news, there is always an anchor fully dressed and made up and ready to report immediately. Do reporters spend 8 hours a day sitting around the studio fully dressed waiting to report 30 minutes of news, or any breaking story that might come up? What about the breaking news that happens in the middle of the night? Are anchors constantly on call, or is there always someone in the studio ready to report? Do they have another job to do while waiting to report a story?