Many modern Windows laptops now come with light sensors that automatically adjust the screen's brightness based on ambient lighting. The feature works inconsistently in Windows, which can make for a frustrating experience. If the brightness switching is driving you crazy, here's how to disable adaptive brightness.
Go to Control Panel > Hardware and Sound > Power Options, then click on "Change plan settings" next to your active power plan.
Click on "Change advanced power settings."
Scroll down to Display, then under Enable adaptive brightness, switch it off for both the battery and plugged in modes.
Adaptive brightness is available in Windows 7 and 8, but will only be visible if your hardware supports it. If your screen still seems possessed, check your display driver for a power saving setting. On Intel cards for example, turn off "display power saving technology."