How does a computer signal via VGA for the monitor to sleep
Using the 15 pin VGA connector, you can signal to the monitor to go into standby (sleep, energy saver, etc) mode by sending the appropriate signals:


  VESA DPMS power saving VESA has defined a standard method for computer to tell monitor when to go to power saving mode. This power saving mode is controlled by changing the sync signals according the table below: 

            NORMAL STANDBY SUSPENDED  OFF 
H-sync        On    Off       On      Off 
V-sync        On    On        Off     Off 
Power level  100%   80%      &lt;30W     &lt;8W



(source: ePanorama: VGA analog display connector)