If your laptop or tablet uses Intel Display Power Saving Technology (DPST) for power plans for your display brightness, the display settings may not work correctly if the device is switched between AC (plugged in) and DC (battery) power while Windows 8.1 is in Sleep mode. This behavior may impact the battery life if the DPST display brightness is not functioning as designed.
This is a known Microsoft issue and is currently being investigated. If Windows is in sleep mode then the power source is changed, Windows does not deliver the new power information to the graphics driver, and it does not update.
Disconnect and then reconnect the AC power while Windows is in the waking state. This will reset the power settings for DPST.