To save credits, you can shut down your server automatically when there are no players on it for a certain period of time. You can configure this in your server options. You can either turn it off completely or set a custom timer.
If it is on you should notice the small timer in the left corner of the green bar on the server page:
This timer tells you when your server will be stopped.
As soon as at least one player is on your server, the timer vanishes and your server will stay online. After the last player has left your server the timer will start again from the beginning.