Why does the LCD screen become dimmer or the noteboook switches off automatically, although it is still powered by the AC adaptor?

Why does the LCD screen become dimmer or the noteboook switches off automatically, although it is still powered by the AC adaptor?
When I leave my VAIO notebook, which is connected to the AC adaptor, for about 25 minutes without touching it, the following symptoms occurs. (The symptoms vary depending on models.)

- Symptom 1
The power indicator lights up in green but the screen becomes dim.

- Symptom 2
The power indicator flashes in orange and the screen becomes dim.

The VAIO notebook equipped with ACPI (Advanced Configuration and Power Interface) for power management is configured at the factory-shipped state to enter into one of energy saving modes if it is left untouched for about 25 minutes, even though it is powered through the AC adaptor.

This configuration adheres to the requirements of the energy saving function set forth by The International Energy Star Office Equipment Program.

- Symptom 1 indicates that the machine is in the Video Standby mode where the display is in hibernation. When user touch a pointing device such as a touch-pad, the screen resumes to normal.

- Symptom 2 indicates that the machine is in the Standby (system suspend) mode where the contents of the memory are retained but the functions of the display, CPU and keyboard are suspended. If press any key on the keyboard, the system resumes to normal. Press the SHIFT key, for example, and wait for a while until the system resumes to normal completely.
Note:
The time necessary for resuming normal differs depending on models and the work conditions before the machine enters into the energy saving mode.

Note:
To resume the system in symptom 2 to a normal state, Sony do not recommend user to press keys that used for "action" or "execution" (for example, ENTER key). This is because these keys may delete data or perform some unexpected operations, depending on the work state, before the machine enters into the energy saving mode.

For VAIO notebooks where PowerPanel is not pre-installed, user can change the configurations of the energy saving mode and the timer on Power Management of Control Panel.
For VAIO notebooks where PowerPanel is pre-installed, user can change the configurations on Edit/Create Profile of PowerPanel.