Regardless of whether you have only one display attached to your Mac or if you have multiple displays, there may be times when you would like to quickly turn them off either for when you are away from your computer or for when you are only using one monitor and do not wish to use the others. Apple offers a few default configuration options for various display setups in OS X, allowing you to adjust when the system puts them to sleep.
Managing all displays
Apple's options for managing displays are a bit scattered around the system and the system preferences, and when set or activated will affect all displays attached to the system. The primary setting is in the Energy Saver system preferences, where you can set the time for display sleep. This same setting can be adjusted by using the following command in the Terminal, where "NUMBER" is the number of minutes of idle time before the display is put to sleep (0 is the same as never):
sudo pmset displaysleep NUMBER
In addition to system settings for automatic display sleep, you can invoke display sleep manually either through the keyboard or by using system hot keys. This is convenient for quickly shutting off displays if you are stepping away from your system, and there are three options available depending on your system setup.
Dim the display (Apple displays only): For any Apple-branded or built-in display you can dim the display by using the brightness controls on the keyboard (usually the F1 and F2 keys). If you dim the display all the way, the last level will shut the display off. While this does not technically put the display in sleep mode, for all intents and purposes it is the same thing.
Hot keys: If you press the Shift-Control-Eject key combination then any displays attached to your Mac will be put to sleep. This is convenient and can be used on any Mac, regardless of the alternative display sleep setups. The only problem is remembering it among numerous other hot-key combinations for OS X.
Hot Corners: In addition to keystrokes, you can set a corner of the display to activate display sleep whenever you move the mouse pointer to that corner. These options can be set in the "Expose & Spaces" system preferences in OS X 10.5 and 10.6, and any of the four screen corners can be used.
Managing individual displays
While the previous options all affect every display attached to the Mac, there are a couple of ways to manage individual monitors attached to your system. The first is to just power them off when not using them, though this does have a couple of drawbacks in that powering them back on may take a couple of moment while the firmware initializes and also that the system will still recognize it as being attached, so windows may be lost in extended desktop modes.
The second option is to simply unplug the monitor, which may be a more practical solution for laptop owners than for others, since the display connection is right on the side of the computer. Unplugging the monitor has the advantage that it will reset extended desktop modes and thereby prevent any windows and items from being lost on the missing external desktop.
Lastly, if you would like to keep your monitors attached and turned on and still be able to put them to sleep, then OS X does not have the ability to do this; however, there are some third-party utilities that can implement this option. One of these is the display manager SwitchResX, which, in its latest version, can enable or disable specific displays either by using its menu extra or by using the display contextual menu.